388 Dr. Reid on the Development of the 
the spawn was placed, is rendered probable by the circumstance 
that they were seen in portions of spawn, in which, to judge from 
other portions of the same spawn examined at a later period, all 
these irregularities disappeared at a more advanced stage of their 
development. The external form of the embryo of the Dendro- 
notus arborescens presented a much greater departure from that 
of the Doris bilamellata than any of the others. Fig. 22 is a re- 
presentation of the left side, and fig. 23 of the right side of the 
embryo of the D. arborescens as it was leaving the case-membrane. 
The shell (fig. 24) was more elongated in the vertical direction, 
the embryo occupied a smaller portion of the shell, and the parts 
which project beyond it were relatively considerably smaller. All 
the textures were transparent, and the retractor muscles were 
very distinctly seen. The membrane surrounding the body (v) 
was attached to the shell around the origin of the retractor 
muscles. When it retired within the shell, the ciliated discs and 
foot were drawn down to a considerable distance from the orifice 
of the shell. When examined at an earlier period of its develop- 
ment, the whole embryo was decidedly shorter and much less 
transparent. I have had no opportunity of examining the em- 
bryos of the D. arborescens, except when developed under artifi- 
cial circumstances, but the embryos possessing the appearances 
described, seemed healthy and active. ) 
To what extent the artificial circumstances under which the ova 
of these animals were kept, influenced the period of time occupied 
in their development, we are not prepared to form an opinion. 
That the changes of structure described are those that occur in 
ova of the Doris bilamellata and D. tuberculata when left in the 
situations where they are usually deposited, was proved by the 
examination of portions of the spawn removed at different pe- 
riods after deposition upon the rocks. The development of the 
ova of the Doris bilamellata proceeded more favourably than 
that of the others; but sometimes a considerable number even 
of these had their development arrested, and otherwise rendered 
monstrous, though supplied daily with water fresh from the 
ocean. I have as yet failed, though I have made the attempt im 
various ways, to keep the embryos alive after they leave the 
spawn, sufficiently long to trace the further stages of their de- 
velopment. Sars* and Messrs. Alder and Hancock} have al- 
ready announced that the young of the Nudibranchiate Mollusca 
undergo metamorphosis, that they swim about for a time in- 
closed in a nautiloid shell, and that at this period they differ 
* Wiegmann’s ‘Archives’ for 1841. I havenot seen Sars’s paper, and quote 
this reference to it from Alder and Hancock. 
+ Report of the British Association for the Advancement of Science, 
volume for 1844, p. 27. 
