Ova of the Nudibranchiate Mollusca. 389 



entirely in their external form and in their organism from their 

 parents. 



I may here mention that three of the Dendronotus arborescens^ 

 which I kept alive at home for nearly a month, often emitted very- 

 audible sounds, which were heard distinctly at the distance of 

 twelve feet. Dr. Grant, who first noticed these sounds, supposes 

 that they are produced by the action of the jaws. They exactly 

 resembled the noise produced by a stroke upon the surface of the 

 earthenware vessel in which they were kept, so that I at first 

 imagined that it might be caused in this way, though it would 

 be difficult to conceive how these animals could strike blows so 

 forcible as to occasion so loud a sound. I however heard these 

 sounds when it appeared to me that the animals were removed 

 from the surface of the vessel and resting upon the branches of 

 some zoophytes. Messrs. Alder and Hancock mention that they^ 

 have frequently kept these animals for several days together with- 

 out detecting the emission of any sound. It is possible that the » 

 animal emits this sound only during the breeding-season. , / 



EXPLANATION OF PLATE X. 



Fig. 1 . Ovum of Doris hilamellata : a, case-membrane (chorion) ; h, yolk or 

 vitelline mass. 



Fig. 2. Part of the vitelline mass evacuated, showing the vitelline mem- 

 brane ; a, small clear cell, sometimes seen attached to outer sur- 

 face of vitelline membrane. 



Fig. 3. Small cells (nuclei) forming the greater part of the vitelline mass. 



Figs. 4, 5. First or bipartite division of the yolk. 



Figs. 6. 7, 8, 9, 10, 11. Subsequent divisions of the yolk. 



Fig. 12. Greatly enlarged view of one of the numerous cells into which the 

 yolk ultimately divides. 



Fig. 13. Embryo on the eighth day after extrusion of the ova. 



Fig. 14. Embryo of Polgcera quadrilineata at the same period of its deve- 

 lopment. 



Fig. 15. Embryo of the Doris hilamellata at the ninth day. 



Fig. 16. Anterior view of the embryo of the Doris hilamellata at the twelfth 

 day. 



Fig. 17. View of right side of the same. 



Fig. 18. View of left side of the same. 



Fig. 19. Posterior view of the same. 



Fig. 20. View of right side of the embryo when it is ready to leave the case- 

 memhrane. 



Fig. 21. View of the same when it has retired within its shell. 



Fig. 22. View of left side of the embryo of Dendronotus arhorescens when 

 about to leave its case-memhrane. 



Fig. 23. View of right side of the same. 



Fig. 24. Anterior view of shell of Dendronotus arhorescens. 



Fig. 25. Lateral view of the shell of Doris hilamellata. 



