388 Dr. Reid on the Development of the 



the spawn was placed, is rendered probable by the ch'cumstance 

 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 lea(!ving 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 difibrent 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 in 

 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 f have al- 

 ready announced that the young of the Nudibranchiate MoUusca 

 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 have not seen Sars's paper, and quote 

 this reference to it from Alder and Hancock. 



t Report of the British Association for the Advancement of Science, 

 volume for 1844, p. 27. 



