48 On the Classification of Nemertes and Planaria. 
ee ee ee ee 
teropoda 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 among Acephala that of Tunicata, it requires 
no effort of imagination to admit in that division, animals such as 
Planarize. They are flattened Molluscs in the same manner as 
Nemertes, are elongated or stretched Molluscs, as are also Denta- 
lium, which nobody would place elsewhere than among Molluscs. 
any authors have spoken of the organization of Planarie, 
from Dugés to MM. de Quatrefages and Blanchard. All have 
viewed them as worms, doubtless —— by the idea -— 
Cuvier who had established the divisions could not have bee 
mistaken so far as to place side by side i in the same order anata 
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 te whole kingdom; and as he 
said there are but four of these 
He who has done most after paves to establish the doctrine 
of these four plans of organization, is Prof. Agassiz. He has 
the unquestionable merit of having followed out the traces of 
these plans beyond the existing creation, and in ascending through 
past ages of the world’s history he has been enabled to recon- 
struct “the biological phases to which I have alluded above. In 
reading the history of the earth on the strata which compose 
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 preconceived by Cuvier, demonstrated as a doc- 
trine by Prof. Agassiz. 
hese 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 “rs 
7 
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Now. the aan respecting the class among Molluscs to which 
Planarie pies is — settled. ‘They crawl on the 
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