302 M iscellaneous, 
quently any deduction 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 different larval forms. 
It is thus that Fritz Miller has shown, by the embryogeny of Peneus, 
the bonds which unite the Nauplius and the Zoéa. 
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 Pilidium, which constitutes one of the most 
typical examples of geneagenesis, numerous larve 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 larve of the Echinoderms. 
From this first sketch originates, by internal budding, the future 
Nemertes, 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 larvee. 
of Desor, which reach their complete development gradually with- 
out presenting any abnormal phenomenon, 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, Wimereux, 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 Pilidium, 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 Pilidiwm to the larva of Desor. 
It is known, from the recent researches of Kowalevsky and 
Metschnikoff, that in the Nemertes with a Pilidiwm the spheres of 
segmentation of the egg arrange themselves very early radiately 
around a central cavity, which is at first very small; this latter 
