302 MtsceUaneous. 



quently any diKluclion drawn from the mode of development 

 becomes impossible, and embryogeny (that powerful aid to anatomy) 

 seems to fail entirely. 



It is therefore of the greatest importance to obtain a knowledge 

 of the mutual relations which unite these ditferent larval forms. 

 It is thus that Fritz Miiller has shown, by the embryogony of Peneus, 

 the bonds which unite the ^^auplhis and the Zo'ea. 



Of all the groups which present this mode of complication, the 

 Nemertians certainly show one of the most remarkable cases. Side 

 by side with the form Pilid'nnn, which constitutes one of the most 

 typical examples of geneagenesis, numerous larva? occur, which, 

 without any analogous phenomenon, pass directly to the adult state. 

 On the one hand we have a transparent animal furnished with 

 elegant extensions and ciliated bands, which the older observers very 

 naturally compared to the well-known larva? of the Echinoderms. 

 From this first sketch originates, by internal budding, the future 

 Semertcs, which, as soon as it is formed, quits its nurse to live an 

 independent life. On the other hand, again, we see a small ciliated 

 very simple larva issue from the egg, a simple oval body, differing 

 but little in appearance from the egg which gave it birth (the larva 

 of Desor), and which, without any other perceptible phenomenon 

 except a mere differentiation of tissues, is gradually transformed 

 into a complete Nemertes. 



During a residence of several months last summer at the Zoolo- 

 gical Laboratory of Wimereux, directed by Professor Giard, I was 

 enabled to study this question in a connected manner ; and it is the 

 results of my researches on this subject that I have the honour of 

 communicating to the Academy. 



Together with a great number of unimportant forms of the larvai 

 of Desor, which reach their complete development gradually with- 

 out presenting any abnormal plieuoraenon, I had the good fortune 

 to meet with some forms of great interest, which, besides a great 

 number of very instructive facts, have furnished me with the 

 transition term between the two modes of development, so different 

 in appearance, the Pilidium and the larva of Desor. 



Among all the species which I have observed, the most remarkable 

 is without question a species very common at "NVimereux, and which 

 I have been able to follow in a very detailed manner in all the 

 phases of its evolution, namely Nemertes communis (Van Bened.). 

 Although reproducing in its development all the essential peculi- 

 arities which characterize the PUidhnn, this species presents a very 

 marked approach towards the simpler states, and offers incontestable 

 analogies to the larva of Desor. 



I reserve for a more extended memoir the details relating to the 

 very curious processes which give origin to the various systems of 

 organs of the Nemertians ; I only desire now to call attention to a 

 main point, the passage from the Pilidium to the larva of Desor. 



It is known, from the recent researches of Kowalevsky and 

 Metschnikoff, that in the Nemertes with a Pilidium the spheres of 

 segmentation of the egg arrange themselves very early radiately 

 around a central cavity, which is at first veiT. small ; this latter 



