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 bilamellata : a, case-membrane (chorion) ; 6, 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 Polycera quadrilineata at the same period of its deve- 
lopment. 
Fig. 15. Embryo of the Doris bilamellata at the ninth day. 
Fig. 16. Anterior view of the embryo of the Doris bilamellata 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- 
membrane. 
Fig. 21. View of the same when it has retired within its shell. 
Fig. 22. View of left side of the embryo of Dendronotus arborescens when 
about to leave its case-membrane., 
Fig. 23. View of right side of the same. 
Fig. 24. Anterior view of shell of Dendronotus arborescens. 
Fig. 25, Lateral view of the shell of Doris bilamellata, 
