ESSAY 
ON THE 
CLASSIFICATION OF NEMERTES AND PLANARIJ!: 
PRECEDED BY SOME GENERAL CONSIDERATIONS ON THE PRIMARY 
DIVISIONS OF THE ANIMAL KINGDOM. 
CHARLES GIRARD. 
I. 
I am gathering materials for a monograph of the Nemertes of this 
side of the Atlantic. I have already said, on another occasion, that I 
■was doing the same with regard to the Planarioe. It will not be antici- 
pating the final results of these investigations, to present here some 
general considerations respecting the place that may conveniently be 
assigned to these animals in our zoological system. 
The chronological history and the position that the various authors 
have assigned to the Nemertes and Planarise having been fully brought 
out by Mr. de Quatrefages,* I shall not dwell upon that part of the 
subject. 
The question now under consideration bearing merely upon the fun- 
damental groups of the animal kingdom, I need only go back to Cuvier, 
since he established these groups, and since the foundations of our 
classification are the results of his labors. 
Nor shall I at present enter upon the secondary divisions to be estab- 
lished among the Nemertes and Planarise ; this aspect of the question 
cannot be settled until we are better acquainted with the organization 
of the group of Rhabdoccelse or fresh-water Planarise. 
II. 
Before, however, I treat of the Nemertes and of the Planarise in a 
* Ann. Sc. Nat., 3d Serie, iv. 1845, p. 129. 2 
more particular manner, I have a few words to say on the subordina- 
tion of characters in the primary groups of the animal kingdom. 
There are in the animal kingdom but two main systems of organs, to 
which all the others are related, being, in a word, mere dependences or 
branches of them. These two systems of organs are: (1,) the nutritive 
system, or system of vegetative life, and, (2,) the sensitive system, or system 
of animal life. 
Now, which of these two systems of organs is the more important; 
in other words, which ranks higher as a zoological character ? The 
nervous system, some physiologists will answer. But let us examine 
this point. What is the distinguishing character of the animal king- 
dom as a kingdom ? A digestive cavity of some kind, into which are 
introduced nutrient substances necessary for the maintenance of living 
bodies ; necessary to their growth anterior to the period of full develop- 
ment, and to their equilibrium after having reached their, complete 
size. The digestive system then is the universal''character’,* common to 
all animals, and this character gives us the animal kingdom ; it ranks 
then highest. 
Next to the entire animal kingdom, we have the four great primary 
divisions, the division of Yertebrata, the division of Articulata, the divi- 
sion of Mollusca, and the division of Radiata; and these four divisions 
are characterized by the nervous system chiefly. The plan of struc- 
ture of the nervous or sensitive system giving these divisions and these 
divisions exclusively, its importance is of a secondary degree. For the 
nervous system has not yet been materially demonstrated in all the 
Radiata, whilst the nutritive system is to be seen everywhere. Not 
that I deny the sensitive system to any animal, even where it has not 
been shown. There exists among the lower Radiata a homogeneity of 
substance, which is perhaps the only obstacle to its discernment; never- 
theless, the digestive system being everywhere distinct, this latter must 
have the pre-eminence. 
It has the pre-eminence because it gives us the unity of the kingdom, 
as we have also this unity in the perfect resemblance of all eggs at an 
early period of their history. In the animal the first substance which is 
formed is the vitellus or yolk ; it is the foundation of the future being; 
it is, as Prof. Agassiz has observed, the being itself. Out of the yolk 
the nervous system originates, as well as all other parts of the organism, 
so that in an embryological point of view the nervous system holds a 
second rank. 
It is of secondary rank as a material organ and character of classifi- 3 
cation of animals. It plays the highest part by its immaterial essence in 
the human species. But here we leave the boundary of the animal 
kingdom, and therefore the classification, which is our object, to enter 
another kingdom, the kingdom of Thought. 
The nutritive system being the index of animality, we see all animals 
equally compelled to take food ; this is the essential condition of their 
existence. 
The nervous system being the fundamental basis of the primary di- 
visions, it gives to each division a special immaterial tendency, so long 
taught by Prof. Agassiz. Now, as there are four divisions, there are 
also four of these tendencies. And as soon as there are four immate- 
rial tendencies, there is an antagonism amongst them. This is a natural 
consequence, since the nervous system overrules the division, and its 
dominion is of a spiritual character. 
The nervous system stamps upon the divisions their zoological form 
as the symbol of their diverse tendency. We shall see further an appa- 
rent anomaly of this kind in the beings placed at the boundaries of two 
divisions, where the material form, to use the words of Milne Edwards, 
escapes the supremacy of the nervous system. The principle, however, 
remains always the remote cause. 
Besides the antagonism of the divisions between themselves, there is 
an antagonism between the instinct and the intellect, that is to say, three 
of the divisions against the fourth ; the Articulata, the Mollusca, and the 
Radiata, collectively known under the appellation of invertebrate ani- 
mals, all of which have only instincts, against the Vertebrata, which 
possess, besides instincts, intelligence to a certain degree. 
Now, between the invertebrate animals and the vertebrate the struff- 
gle is latent, passive, because the two principles, the instinct and the 
intellectnood, cth exist. But among Vertebrata, where we find both 
the instincts and the intellect in the same individual, the struggle is ac- 
tive and direct, and we are indebted to the observations of Fred. Cuvier, 
for the revelation of this astonishing law, that wherever the instincts 
command, the intellect is actionless, and wherever the intellect governs, 
the instincts are silenced or nearly so. There is a struggle and an open 
struggle ; the victory of one of the principles involves the subjection of 
the other. 
Morally speaking, we might thus establish two series in the animal 
kingdom ; an instinctive series, stationary and sightless, and an intellec- 
tual series, progressive and seeing; the first including three divisions, 
(Radiata, Mollusca, Articulata,) and the second only one ( Vertebrata.) 4 
And now, if we go back to the origin of the animal kingdom, and 
trace its history in the past, we see the two series appearing simultane- 
ously from the first manifestation of life upon the surface of the earth : 
the instinctive series with its three divisions and all the classes; while 
the intellectual series is represented by one class only, that of fishes. 
Their forms are renewed dui'ing a succession of periods, and each great 
revolution of the globe adds one class to the division of Vertebrata or 
the intellectual series, in the order of their zoological gradation : Rep- 
tiles, Birds, and Mammals :—then Man crowning the whole work. Thus 
a real progress is manifested in the division of Vertebrata, while the In- 
vertebrate animals remain what they were, although undergoing a re- 
newal of forms. The reason of this is, as Fred. Cuvier states, that the 
instinct is innate, always sightless, necessary, and unchangeable, whilst 
the intellect is progressive, conditional, and susceptible of modifica- 
tions.* 
In the intellectual series there was an aim, a design, and this was, to 
arrive at man, the true domain of intelligence. This aim realized, the 
creation would stop, and it did stop. Zoological forms had acquired all 
that diversity with which the sphere of activity of each division was en- 
dowed. To the immaterial principles nothing was left except a limited 
play, a contest for supremacy. To intelligence alone was given the 
power to arrive at the knowledge of the actual world, to look back in 
time, to contemplate itself in the past in view of the future, finely to 
study itself,—in a word, to reflect * 
The power of reflection belongs exclusively to man, the last being 
created. Man being the converging point of the material creation, in 
him were also to be concentrated in our time the struggles of the two 
spiritual principles of all past times. 
One word more on the intellectual series. The fishes, reptiles, birds, 
and mammals belong to this series; but the fishes, the reptiles, the birds, 
and most of the mammals, in their natural condition of life, have no in- 
telligence,—have no intellect. 
The intellect resides within the brain, and the Vertebrata alone have 
a true brain. The brain is composed of several parts. There is the 
base of the brain, which sends nerves to the organs of sense, and the 
hemispheres, the special seat of the intellect. Now, of the hemispheres, 
the fishes have only a rudiment, and this is the reason why they have 
* Flourens, Resume des travaux de F. Cuvier, sur l’instinct et l’intelligence de» 
animaux. Paris, 1844. 
f Ibid. 5 
no intellect. There exists a well defined progression from the 
fishes to the mammals with respect to the development of the hemi- 
spheres ; placed anteriorly in the fishes, they rise degree by degree in the 
other classes over the base which is gradually covered and concealed 
under them. Here we see the organ reflected upon itself, reminding us 
of its function in its full activity, reflection. To this gradual develop- 
ment corresponds a position of the head more and more raised, which 
becomes vertical in man, where it forms a right angle with that of 
fishes. One step more would have been retrograde : the development 
there stopped. 
Thus, by a gradation almost imperceptible, we have beings belong- 
ing to the intellectual series which have the intellect only in a virtual 
state. They have the organ without having the principle; or at least 
admitting the principle virtually present, the organ is not sufficiently de- 
veloped to allow its manifestation. 
These general considerations, although a brief resume, will perhaps 
appear out of place in this paper ; but my object is the discussion of the 
value of the nervous system as a zoological character, and to show that 
while this system of organs gives only the divisions, these latter are 
governed by it in an absolute manner. 
I now come to the special topic of my communication. 
III. 
The place assigned by Cuvier to the Nemertes and to the Planarise 
in his “ Animal Kingdom,” is entirely provisional, as acknowledged by 
the illustrious naturalist himself. 
The Nemertes are placed in the division of Radiata, immediately after 
the Inteslina cavitaria. 
Between Nemertes and the intestinal worms of this order there are 
only analogies. The extraordinary length of the body of some of them, 
for instance N. Borlasii (Borlasia angliae), a length reminding us of the 
class of worms, and, above all, of some of the intestinals, such as the 
tape worm, had prevailed over all other considerations. Their affinities 
were not acknowledged, because their organization was unknown. 
At that time the intestinal worms were regarded as Radiata, for the 
reason that their nervous system had not been found, and the Nemertes, 
as well as Planai-iae, were regarded as intestinal worms, because all of 
them reminded us, by their forms, of the forms of these last. 
When the more recent labors of some zoologists had established 6 
beyond any doubt that the intestinal worms belonged to the division of 
Articulata, on account, first, of their having a nervous system, and a 
nervous system constructed on the plan of that group, secondly, by the 
structure of their body, which is composed of a series of articulations 
or rings movable upon each other, then the Nemertes were carried with 
the Inteslina cavitaria into the division of Articulata, where they re- 
mained as little known as before. It is but of late that they have been 
made the subject of a special study by a skillful zoologist, Mr. de Quatre- 
fages, and I am surprised that this author has not pointed out the close 
affinities which they bear to Mollusca. 
Cuvier was well aware of the space which separated Nemertes from 
intestinal worms, inasmuch as he foretold that they would one day con- 
stitute a new order. In spite of these external resemblances, their 
structure, which as he says “ is of an extreme softness,” caused him to 
doubt. Nevertheless it did not, on this account, enter into his mind to 
compare them with molluscs. 
At that time, as indeed now, the idea of a mollusc corresponded with 
the idea of a shell-bearing animal, with the form of a body more or less 
drawn together into itself, while the lengthening of the body involved by 
analogy the idea of a worm. 
Now if, abstracting the form, which is not the characteristic of the 
divisions, we look at the intimate structure, if we give up also the shell 
as circumscribing the division of Mollusca, we shall find in the Nemertes 
all the principal characters of molluscs: a soft body, entirely smooth, 
covered with a glutinous mucosity ; a very simple nervous system re- 
duced to a small number of cephalic ganglia, whence nervous threads 
depart to distribute themselves in the body. If we further state that, 
as in the greater number of molluscs, the surface of the body is covered 
with vibratory cilia, which help their movements, movements generally 
slow, deprived of energy, then we directly arrive at the idea that Ne- 
mertes are really molluscs,—molluscs of a low rank, being parallel 
with the worms of the division of Articulata by the analogy of their 
forms. 
Having discussed above the value of the nervous system as the 
predominant character of the division, exclusively of any other character, 
it remains only for me to state that the nervous system of Nemertes is 
constructed upon the plan of the nervous system of molluscs: there 
exists a cephalic mass, more or less lobed, representing either the superior 
sesophageal ganglion of other molluscs, or that same ganglion to which, 
on account of the peculiar form of the body, are added the two or three 7 
abdominal ganglia. Nervous threads are distributed in all directions ; 
two of them, more voluminous than the rest, but uniform in structure, 
run along the sides of the animal, sending off thinner threads, without 
showing in their course those ganglia or swellings which distinguish 
the nervous system of Articulata, such as it is in Malacobdella, Peri- 
patus, &c. 
The disposition of the nervous system of Nemertes, then, is merely 
analogous to that of annelids ; its structure is that of the nervous sys- 
tem of molluscs. 
The position of Planarige in the division of Radiata is not less curious 
than that of Nemertes. Included in the second order of intestinal worms, 
the f’arencliymata, they are brought near the Trematodes, to which 
they have only analogies, in the same sense as those that Nemertes have 
to the Intestina cavitaria. 
The Intestina parenckymata have been withdrawn from the division 
of Radiata and brought into that of Articulata, and for the same reasons 
as that of the cavitaria. The Planarise, of course, have thus been com- 
pelled to follow them in the same manner as Nemertes have followed 
the latter. But also little investigated at that time, their affinities with 
molluscs have escaped the eyes of zoologists. 
The Planariae are not parasitical, as Trematodes are, and it is 
important that this fact should be noted, parasitism existing most 
extensively among Articulata. The investigations of Mr. de Quatre- 
fages and others have moreover made us acquainted with their 
structure, which, although not yet entirely understood, as I believe, 
is however of the highest interest. Indeed, in Distoma the digestive 
system is constructed upon an analogous plan with that of Planaria; 
but the digestive system, as we have said, cannot characterize the 
primary division. It characterizes the kingdom. 
As soon as it is acknowledged that the division rests upon the structure 
of the nervous system, the fact that the nervous system of Distoma is 
that of Articulata, and the nervous system of Planarise is that of Mol- 
lusca, there is no ground for further hesitation. 
In Distoma there are two cephalic ganglia, whence nervous threads 
depart for the anterior region of the body. From each one of these 
ganglia arises a nervous thread, which takes its course backwards; this 
thread presents on its length a series of small ganglia, scarcely distin- 
guishable upon the middle region, it is true, but very apparent towards 
IV. 8 
the posterior region, where they are seen sending off smaller threads, dis- 
tributing them to the body. This arrangement of the nervous system 
of Distoma becomes especially distinct in Malacobdella, where the same 
arrangement is found, with the ganglia of the lateral threads more 
developed. Thus, taking the nervous system into consideration, 
Malacobdella is one degree higher than Distoma ; then Clepsine would 
follow, in which the two threads are brought so close together that they 
combine into one single thread. Above Clepsine would rank the other 
Hirudines. 
In Planariae we have a cephalic ganglion, more or less lobed on its 
circumference, which sends nervous threads to all the regions. There are 
two more voluminous lateral ones (one on each side of the body), as in 
Distoma, but uniform, as in Nemertes, still recalling here by analogy the 
nervous system of Articulata. The fundamental difference, although 
less apparent at first sight, consists in the absence of ganglia upon their 
lengths, and this fact decides all. 
The body of Distoma, indeed, is not articulated, and this may 
perhaps lead to a belief of a closer affinity between Planariae and 
Worms. Their broad and flattened form was better adapted to their 
mode of life without that structure. Moreover, the articulation of 
the body, although a character of the division, is subordinate to tha 
nervous system. Consequently, there is no ground for surprise at 
seeing it vanishing or even entirely disappearing in those groups 
placed at the confines of two divisions as a material litigious property 
disputed by antipathic vital tendencies, each endeavoring to appropriate 
it to itself. 
Considering now the softness of Planari®, that glutinous mucosity 
which surrounds them, that body of a uniform shape without articulations 
and deprived of articulated limbs, that general apathy, all these are so 
many characters which they partake with Mollusca, and do not partake 
with Articulata. 
However, I do not know that any one has proposed to consider 
these animals as Mollusca, aHhough Baer and Duges had already 
compared the inferior disk of Planariae to the foot of Gasteropoda, 
without thinking of bringing them together in the same natural group. 
Mr. de Quatrefages refutes this comparison. We shall come to it again 
presently. 
For, if the shell does not characterize Mollusca, inasmuch as all living 
Cephalopoda are naked, and among Gasteropoda, we have the whole 
group of Nudibranchiata deprived of a shell, that of Pteropoda, and 9 
among Acephala that of Tunicata, it requires no effort of imagination to 
admit in that division animals such as Planarise. They are flattened 
molluscs, in the same manner as Nemertes are elongated or stretched 
molluscs, as are also Dentalium, which nobody now would place else- 
where than among Mollusca. 
Many authors have spoken of the organization of Planariee, from 
Duges to MM. de Quatrefages and Blanchard. All have viewed them 
as worms, doubtless prepossessed by the idea that Cuvier, who had 
established the divisions, could not have been mistaken so far as to 
place side by side in the same order animals belonging to two different 
divisions. 
. But this error of the author of the “ Animal Kingdom” is easily 
accounted for. At that time he was in want of the essential datum 
to settle such a question in its details, the knowledge of the nervous 
system. 
The plan of structure of the nervous system, we have already 
said, and we cannot repeat it too often, gives only the division and 
nothing but the division. Now, Cuvier did not know it when he laid the 
foundation of his classification; although he foresaw the four plans of 
organization of the whole kingdom ; and, as he said, there are but four 
of these plans. 
In reading the history of the Earth on the strata which com- 
pose its crust, as so many pages of a book written by the hand of 
the Creator, we then find again the thought of these four plans of 
organization. 
These four plans of organization would acquire a far greater 
importance if embryology should ratify them. Now embryology 
does so. All embryological investigations, past and contempora- 
neous, lead towards four plans of structure. I shall not treat of 
this question more in detail here ; it is sufficient merely to mention 
the fact. 
Now the question respecting the class among Mollusca to which 
Planariae belong, is easily settled. They crawl on the inferior surface 
of their body; that they are gastropods, there can be no doubt. Do 
we not see them, as well as Pulmonata, Nudibranchiata, and others, 
creeping along the walls of a basin, and, when near the surface of the 
water, reversing their position and walking in that manner with the same 
facility ? Do we not witness the same undulatory contractions in that 
foot ? Therefore, were not Baer and Duges guided aright when they 10 
compared the inferior surface of the body of Planarise to the foot of 
gastropods ? 
I am not aware that any of the Planarise I have studied moves with 
the same facility on the back as on the belly, as Mr. De Quatrefages 
states. Whenever I placed them in that position, I always saw them 
changing it as quickly as possible. Besides, in most cases I have seen 
the upper surface differing widely from the inferior one. 
In Planocera we have on the dorsal surface two cylindric tentacles, 
analogous to the cephalic tentacles of doridians. In Thyzanozoon or 
Eolidicera, the cephalic tentacles are flattened, and are brought entirely 
forward, while other tentacles appear on the back, reminding us of the 
dorsal appendages of eolidians proper. Then come Proceros and 
Procerodes, with but the flattened anterior tentacles of Eolidicera, and 
in the same place as in these last. 
There exist, then, in the marine Planarise one group which reminds 
us of Doris, another of Eolis, and still another slug-like group, the 
Procerodians, intermediate between eolidians and the fresh-water 
Planarians proper. 
This shows that it is near Nudibranchiata that Planarise will find a 
natural place, and on several accounts we should be tempted to consi- 
der them as a degradation of that type. 
But for us, who do not admit the so-called degradations, inasmuch as 
each being appears to us, as to Cuvier, a perfect whole by itself we con- 
sider Planarise as a family nearly allied to that of Nudibranchiata, bear- 
ing in itself the reason of its existence as strongly defined as this latter, 
and representing merely a variation in the Thought of the Creator. 
For, in Nudibranchiata we find a number of types all as much di- 
versified. These are : Doridians, Eolidians, the slug-like Canthopsis 
analogous to Procerodians, and which leads to the Acteonians, then 
finally the genera Pella and Chalidis, which constitute another group 
almost planarian, deprived of external appendages, analogous to the 
terrestrial species of Planaria. 
There exists a striking parallelism between the zoological forms of 
these two families. 
Viewed in the light of their organization, nothing is more alike. The 
nervous and digestive systems scarcely differ. I have already spoken 
of the first. With regard to the second, I shall recall the fact that it 
is ramified in Nudibranchiata as in Planarise ; the ramifications being 
diversified according to the groups. 
No respiratory organs, properly so called, in either Nudibranchiata 
r Planarise. 11 
The organs of generation do not differ much more. Nudibrancliiata 
O O 
are androgynous like Planarise. The fecundation takes place by mutual 
impregnation, as is the case in Pulmonata. But in Planarise we have 
cases where an individual fecundates itself, the hermaphroditism being 
here complete. 
Y. 
It still remains for me to make some general remarks upon Nemertes 
and Planarise. 
Mr. De Quatrefages tells us himself: “ Neither Nemertes nor Plan- 
arise have externally a resistant and tough layer, similar to that which 
is found with annelids, for example, or even with Rotatoria.”* 
We have, then, two groups of gastropodous molluscs parallel with 
two groups of annulated Articulata ; the group of Planarise reminding 
us of Helminthes, and the group of Nemertes reminding us of the 
Hirudines. 
At the extremes of these two groups, at the bottom of the two 
classes, we witness in some sort a strange and opposed struggle of the 
two immaterial principles of the divisions which exert themselves to 
take froip. each other some portion of their material property. As ex- 
amples, we have : 
In Mollusca,—the Nemertes, which elongate and become worm-like; 
the Planarise, which remain shorter but pressed down, spread out, flat- 
tened in thin leaves. 
In Articulata,—The softness of Helminthes in general, the flattening 
of their body in Trematodes, in which the articulation of the body 
vanishes, analogous to Planarise ;—the softness still of the Leeches 
with a distinct articulated structure, being parallel with Nemertes. 
These groups do not oppose each other in an exact parallellism ; for 
Nemertes, which form a low type of gastropods, are opposed to the 
Leeches of a higher grade of worms, and Planarise, a higher grade 
among gastropods, are opposed to Trematodes, a lower type among 
worms. In this manner : 
Hirudines. 
JPlanariae. 
Trematodes. 
.Nemertes. 
In the elongation of the body of Nemertes there is nothing to sur- 
* Am. Sc. Nat., 3d series, vol. iv. 1845. 12 
prise us. Placed at the bottom of the class to which they belong, they 
assume a form analogous to a group of the division of Articulata which 
attract them, but to which they do not belong. Now, when a mollusc, 
whose body is elongated beyond all proportion, and obliged sometimes 
to move by the contraction of transverse muscular fibres, accelerates 
its progress, then we see that mollusc assuming certain transverse, irre- 
gular, and unequal folds—shadows of articulations which in reality do 
not exist. 
At the bottom of the division of Articulata we observe similar facts. 
The Trematodes lose insensibly that elongated form of the body which 
constitutes the prominent character of the worms ; they are flattened, 
spread out, meanwhile the articulation of the body, the characteristic of 
their type, vanishes completely,—thus foreshadowing the type of 
Planarige. 
The position and number of eye-specks in Nemertes and Planarise 
indicate also a greater resemblance to the same organs in molluscs than 
to those of annelids. When eyes exist in annelids, they are arranged 
in pairs on both sides of each articulation, or else form a crown on one 
of the anterior rings. In Planarise we find the eye-specks irregularly 
grouped on the upper surface near the anterior region of the body. 
The same arrangement is observed in Nemertes. This arrangement 
forcibly remind us of what we see in gastropods. In Planarise alone 
they are more numerous, and distributed with less constancy, a fact 
which is accounted for by the lower position of that family in the class. 
The habits of Nemertes and Planarise speak more strongly in favor 
of molluscs than worms. Most of the species live concealed under 
stones, which is the case with many molluscs, while I do not know any 
which lives within a tube constructed like those of worms, even in the 
most elongated Nemertes. 
Moreover, in the southern hemisphere there exists a group of terres- 
trial Planarise, whose habits and characters are, if possible, still more 
molluscoid. They were observed and described bjr Charles Darwin, 
who thus expresses himself on the subject. “ They may, however, form 
a section of the genus [Planaria], being characterized by a more convex 
and narrow body ; their more distinctly defined foot; their terrestrial 
habits ; and frequently by their longitudinal bands of bright color. 
From their colors, from their convex bodies, from their manner of 
crawling and the track of slime which they leave behind, and from their 
places of habitation, they present a striking analogy with some terres- 
trial gastropods, especially with Vayinulus, with which snails I have 13 
several times found them associated under stones.”* This shows not 
only a striking analogy, but the greatest affinity with Molluscs. Those 
air-breathing Planarise, with their convex bodies, immediately recall to 
our mind the terrestrial slugs; and were it not that they want the 
cephalic tentacles of the latter, and possess a ramified intestine, they 
would at once have been placed among Molluscs. If land-slugs have 
a simple intestine, in the it is ramified, showing that on this 
ground alone they could not stay among worms. 
The embryonic development of both Nemertes and Planariae takes 
place according to the laws we witness among gastropods. Even in 
the earliest history of the eggs, we observe similar phenomena in Ne- 
mertes and many gastropods. These pouches, in which several eggs 
or vitellus are enclosed, do we .not observe them in both groups ? The 
motion of the embryo within the egg-envelope, is it not seen in Ne- 
mertes and Planarise, as well as in Nudibranchiata, and other Gastro- 
pods ? The cilia which surround the embryo exist similarly in all of 
them. And the fact that gastropods and Planarise pass through a 
larval state before they reach their full growth and resemblance to their 
parents, is another circumstance which ought not to be overlooked. 
It is plain enough, Nemertes and Planarise are only analogous to 
Articulata; by their affinities they are Mollusca. The division of Arti- 
culata hence appears to us as a type more natural and rational, as well 
as that of Mollusca; this latter including all animals which are soft, 
slimy, and flabby, whetever may be their form. 
The Rhadobcoelae, or fresh-water Planariae, will be the connecting link 
by which the Nemertes approach Planarise proper, as members of the 
same group. We may say of them, that they are the fresh-water re- 
presentatives of both Nemertes and Planarise. And now observe the 
transition: this group, Rhabdoccelae, which is distinguished at once 
from Planarise and Nemertes, this group, I repeat, lives in fresh waters, 
while the other two groups, which they connect together, are scarcely 
found beyond the boundaries of the seas. If some of them ascend the 
rivers, it is within the limits where the water still retains a part of its 
marine character. So that the connection, although materially ex- 
pressed, exists essentially in their immaterial essence. 
VI. 
I now conclude by a few words on Annelids and Gastropods. 
* Annals and Mag, of Nat. Hist, xiv., 1844, p. 242. 14 
Having withdrawn from the first of these classes a certain number 
of its representatives to include them among the second, it is to be ex- 
pected that I should present in a synoptical manner the systematic 
modifications resulting from it. 
If we admit the two sections of worms proposed by Milne Edwards, 
the Pleuronera and Annelids proper, we should place them one above 
the other, to form a single series instead of ifrhD ♦ 
Opposite we should have the series of gastropods, beginning with 
Nemertes ; above these the Planariae ; then the Nudibranchiata. 
I 
U 
o 
£ 
Annelids. 
Annelids proper. 
Scoleides. 
Hirudines. 
Nudibranchiata. 
rj. 
h3 
c 
p 
o 
D 
2 
O 
' Planariae. 
Pleuronera. 
f Peripatus. 
Malacobdella. 
Polyeladus. 
[ Helminthes. 
Nemertes. 
What would come next to the Nudibranchiata I am not prepared to 
say. The series cannot go on, on that footing, for the very simple 
reason that embryology assigns a lower rank to all gastropods pro- 
vided with a shell, inasmuch as Nudibranchiata, when hatched, have a 
shell, which they lose at a later but still an early period of their life. 
The attempt at forming organic series in the animal kingdom is one of 
the most difficult of labors, and a work of a relative value. Unless 
several series are established in the class of Gasteropoda, the position 
of Nudibranchiata in the above synopsis cannot be accounted for. I 
have already made the remark that Planariae were rather parallel to 
Nudibranchiata than of a lower rank, finding in these two families 
groups of equal importance. 
Respecting the serial arrangement of worms, as given above, and 
resting upon the morphology of their nervous system, I have also to 
remark, that it does not coincide with another series alluded to in a 
remarkable paper of late publication ;* and, indeed, if lumbricine an- 
nelids and Leeches are higher in their class, as I should be willing to 
believe, this would corroborate the idea, that the nervous system 
* The natural relations between animals and the elements in which they live, 
by Prof. L. Agassiz. SilUman’s Journal, 2d ser., vol. ix. p. 369. 15 
characterizes only the primary divisions, and cannot express exclusively 
the organic gradation of the natural secondary groups, unless in an 
artificial method. Thus our position taken at the beginning would 
turn out to be the right one. 
Now if, instead of a single series, or several series, with a starting 
point and a terminal end, we construct a circle or an ellipse, and place 
the gastropods on its circumference, then the respective arrange- 
ment of each group would meet with less difficulty, and, perhaps, 
satisfy completely our mind ; for viewed in that light of indepen- 
dent and circular groups, the Creating Thought presents itself to us 
harmonizing with our notions on the whole universe. 
Our constant aim in the study of natural history consists in a thirst 
for Truth, in an aspiration for pure knowledge. 
The natural Truth is defined : the relations which exist between the 
Creator and the created beings ; and we may say more correctly, the 
immaterial relations, &c. 
Now, the creation being the manifestation of Thoughts, under an 
earthy coating, when we dare to ascend to the primitive Thought that 
has called into existence the whole universe, and contemplate the 
Creator’s mind in the work of his creation, we feel ourselves in the pres- 
ence of a majesty almighty in power. No beginning, no end to be 
perceived in Him, as theology teaches us. We witness a material be- 
ginning and a similar end to all living beings, such is their destiny on 
earth. This destiny was strictly defined, their plans of structure clear- 
ly conceived, before they were called into any sort of manifestation, 
into material existence. 
Thus two events strike us most, the so called Birth and Death, cir- 
cumscribing an area within which we observe phenomena taking place. 
This area constitutes the circle of activity with which living beings are 
endowed to cross the present world. But where was the being before, 
and where it is to be afterwards, and how ?* There is a circle of ac- 
tivity for each individual, and for each sex when separated ; there are 
as many circles of activity as natural groups to which the individuals, 
the species, the genus, the family, &c., belong, according to their 
rank. 
Without identifying the individual phenomena of the physical world 
* These questions have certainly their importance to the philosophers of our 
century. During the last one they were discussed by all thinking men, when of 
sound facts we had but few, and the subject was therefore too premature. I have 
much more to say with regard to them, and intend to do so on another occasion. 16 
with those observed in organized beings, we may find out in the various 
orbits and different systems of the sidereal hierarchy an equivalent of 
the same fundamental idea or law. 
The natural groups, therefore, exist in nature, and are not mere fan- 
cies of our imagination, except in the case of artificial systems of classi- 
fication. 
The circle or the sphere, moreover, is the emblem of the intellect 
reverted upon itself; the emblem of the reflexion in its self-possession, 
in its self-knowledge, in its free exercise, in its wisdom. The emblem also 
of the sublime omnipresence of our Creator.