Classification of Sponges. 411 



recognized by this or that histological structure : it is a propo- 

 sition contrary to the very definition of homologies ; they are 

 predestined to play such and such a role in development : 

 this is metaphysics. 



The circumstances appear under a totally different 

 aspect, and no longer lend themselves to any obscurity, 

 if, following the facts without attributing to them any 

 mysterious significance, we remember that the primitive 

 form of every embryo * is not a gastrula composed of two 

 layers, as is gratuitously supposed, but a hlastula more or less 

 approaching the ellipsoidal shape, and constituted by a single 

 layer of cells. This larva is ciliated, and the strokes of the 

 cilia impel it in a fixed direction ; it therefore possesses an 

 anterior and a posterior pole. From the moment at which 

 locomotion is accomplished in a definite direction the cilia of 

 the anterior region of the hlastula are necessarily those which 

 exhibit the maximum of activity ; their activity exhausts the 

 alimentary reserves of the elements which bear them, while 

 the elements of the posterior region remain crammed with 

 these reserves. The elements which are disburdened of these 

 reserves are always those the multiplication of which is the 

 most active. 



This being granted, the hlastula can follow only two courses 

 in its evolution — either it remains free or else it becomes 

 fixed. If it remains free its locomotor anterior region, by 

 reason of its rapid growth^ necessarily induces the invagina- 

 tion of the posterior region, which is essentially nutritive, &nA 

 this is why the orifice of invagination, which has for so long 

 been erroneously considered as a primitive mouth, is very 

 generally posterior in the Nephridiate series (" dans la s^rie 

 des Ndphridi^s "). If, on the contrary, the hlastula becomes 

 fixed, it does so necessarily (the exceptions to the rule in the 

 animal kingdom are only apparent) by its anterior locomotor 

 region. Indeed, an animal cannot adhere to an obstacle 

 except by pressing against it — that is to say, by applying 

 against it the region which, in its habitual mode of loco- 

 motion, it carries in front. It is therefore the ciliated region 

 of the hlastula that in this case will be covered by the granular 

 region, will proliferate, multiplying its anfractuosities on the 

 inside of its envelope, and will constitute the entoderm : thus 

 is to be explained the particular case of the Sponges, which 

 henceforth there is no need to contrast with the rest of the 

 animal kingdom J they detach themselves from the common 

 branch because they become fixed in the 6/as<M^a state ; in 

 respect to this they diverge from the Polyps, since in the 

 latter the cavity of the hlastula, already filled with elements, 



* Except in the Arthropod series, where the vibratile ciha are absent. 



