Miscellaneous . 303 



enlarges rapidly and drives all the cells towards the periphery, so aa 

 to constitute a superficial membrane. There is thus produced a 

 closed vesicle, with the wall formed of a sinj^le series of cells (JAas- 

 toxphrra). This vesicle becomes invaginated and gives origin to a 

 double-walled sac (Gdstrula) ; it is at this stage that hatching takes 

 place. The Gastrida breaks through the vitelline 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 P'di- 

 dium. It is only after this interruption, corresponding to the duration 

 of independent life, that the development commences which is to lead 

 to the formation of the Nmnertes. There is here, evidently, an 

 exaggeration of a larval state followed by a return to the type. 



To form the Nemertes [from the FilkUnm], 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 lamellae of 

 the disks, will become the skin of the Nemertes ; the outer one, formed 

 by the coalescence of the external lamellae, 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 Gastrida. In the same way, the formation of the Nemertes. 

 is accomplished, in general, by means of the envelopment of the 

 intestine by large discoidal lamellaj, which become confluent and 

 unite by their edges to constitute the skin of the Nemertes. Finally, 

 the primitive exoderm is destroyed, and the tnimal formed in its 

 interior is set at liberty. But there the t^nalogy stops. Our 

 Nemertes, in fact, presents some important peculiarities 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, iu the interior 

 of the e^^, 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 simpUficatiou 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 Gustralu covered with fine vibratile cilia. 



21* 



