Miscellaneous. 303 
enlarges rapidly and drives all the cells towards the periphery, so as 
to constitute a superficial membrane. There is thus produced a 
closed vesicle, with the wall formed of a single series of cells (blas- 
tosphera). This vesicle becomes invaginated and gives origin to a 
double-walled sae (Gastrula) ; it is at this stage that hatching takes 
place. The Gastrula breaks through the vitellime membrane and 
begins to swim freely in the liquid. Then commences an interruption 
in the development, during which the larva, adapting itself to pelagic 
life, acquires all the different peculiarities characteristic of the Pili- 
dium. Itis only after this interruption, corresponding to the duration 
of independent life, that the development commences which is to lead 
to the formation of the Nemertes. There is here, evidently, an 
exaggeration of a larval state followed by a return to the type. 
To form the Nemertes {from the Pilidium), four little invaginations 
take place at the expense of the exoderm; these detach themselves 
and produce four vesicles which fall into the cavity of the body of the 
Pilidium, where they become flattened and are transformed into hollow 
disks, formed of a thin external lamella turned towards the exoderm, 
and a thick internal lamella turned towards the endoderm. These 
four disks soon meet, surrounding the intestine, join together, and 
coalesce, and thus form a double membrane around the intestine : 
the inner membrane, formed by the-junction of the inner lamellé of 
the disks, will become the skin of the Nemcertes; the outer one, formed 
by the coalescence of the external lamelle, will constitute a provisional 
membrane, the amnios, which will disappear at the same time as the 
skin of the Pilidium to set the Nemertes at liberty. 
Without being actually identical, the resemblance of the develop- 
ment of our Nemertes to that which we have just indicated is great 
enough to exclude all confusion between the two forms described. 
As before, the first stages of development are characterized by the 
presence of a blastosphere which becomes invaginated to give origin 
to a Gastrula. In the same way, the formation of the Nemertes 
is accomplished, in general, by means of the envelopment of the 
intestine by large discoidal lamella, which become confluent and 
unite by their edges to constitute the skin of the Nemertes. Finally, 
the primitive exoderm is destroyed, and the animal formed in its 
interior is set at liberty. But there the analogy stops. Our 
Nemertes, in fact, presents some important yeculiarities which 
remove it from the Pilidium to bring it nearer the larva of Desor. 
We have, in the first place, the absence of pelagic life and of the 
interruption of the development which results from it. Here all 
the development is performed, from beginning to end, in the interior 
of the egg, and the animal which issues from it has already acquired 
the characteristic form of the Nemertes. Besides this fundamental 
fact, we see also that there is an evident simplification of the 
embryogeny and a gradual progress towards the extreme conden- 
sation which is observed in the larva of Desor. The stage which 
corresponds to the Pilidium has already lost all the different 
characteristic appendages which result from life in a free state, and 
is reduced to a simple Gustrula covered with fine vibratile cilia. 
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