Desor on the Development of Polynoe. 15 
ing that the embryo always escapes tail foremost, this being 
completely free, whilst the head and eyes are within the en- 
velop, (fig. 42.) Around the embryo are seen opaque grains 
of yolk, which remain in the envelop when it is cast off, 
(fig. 43 a.) As soon as the embryo is completely free, 
(fig. 43), it begins to creep, but its motions are very sluggish 
when compared to those of the envelop.' 
I could not succeed in prosecuting the inquiry farther, since 
the embryos fell a prey to a species of Infusoria (Leucophrys), 
which is very voracious, and was at that time very numerous. 
CONCLUSION. 
It follows, from the above statements, that the embryonic 
development of Polynóe agrees with that of the Nemertes in 
the very point we have considered as the most prominent, 
viz. the fact that there is an envelop which, after having for 
a time protected the progeny, and afforded to it the means of 
revolving, is cast off, thus allowing the embryo to become 
entirely free. And as this mode of development is quite 
different from that observed in the other classes of Articulata, 
especially the Insects and the Crabs, we may fairly expect that 
when more extensively investigated, the embryology of the 
worms will afford us the means of a better and more natural 
classification than that which is derived merely from the 
anatomy and external features of the full grown animal.’ 
This is not the place to discuss any of the general ques- 
tions to which these investigations into the embryology of the 
lower animals give rise, if considered in a philosophical point 
of view, especially in regard to animal individuality. Hiti- 
1 A somewhat similar development has been observed by M. Siebold in the em- 
difference, that tbe E seems to be more highly organized, “having even eyes (a) 
that are thrown off wit : 
? Thus we shall have occasion to show, in a future paper, that Planaria, which is 
generally ranked by the side of the Nemertes, is totally different in its embryology 
