Ova of the Nudibranchiate Mullusca. 385 



bodies were seen moving backwards and forwards in the interior 

 of the largest of these two cells*. On the right side there is an 

 irregularly-shaped aperture (fig. 16 ^) immediately behind the ter- 

 mination of the intestine, and between the upper part of the body 

 and the aperture of the shell, through which the water passes into 

 the interior, and cilia were seen in active motion in this situation. 

 A band passes from the upper part of the mass of cells placed at 

 the termination of the intestine, round the neck of the embryo, 

 close to the margin of the aperture of the shell, and forms the 

 outer boundary of this opening by which the water passes into 

 the interior of the shell t- As the largest of the cells, placed at 

 the termination of the intestine, was seen to contract at irregu- 

 lar intervals, I imagined that it might be a rudimentary heai-t, 

 and the band to be a vessel leading from it, but I obtained no 

 satisfactory evidence of the accuracy of this supposition. A py- 

 ramidal-shaped mass projects from the upper and back part of 

 the body (fig. 20 s), from the apex of which a thin membrane 

 descends and passes round the body (fig. 20 1;). A strong band 

 of contractile fibres is attached to the lower part of the left side 

 of the shell, and passing up on the same side divides into two 

 portions, which terminate upon the back part of the neck and 

 gullet. A very minute band passes from the same part of the 

 shell to the lower part of the stomach. These muscular bundles, 

 though distinctly seen, especially the strong band passing up- 

 wards, in the embryos of all the Nudibranchiate Mollusca ex- 

 mined, are remarkably distinct in that of the Dendronotus arbo- 

 rescens (figs. 22 and 23 jojo J). It is by the contraction of these 

 muscular bundles that the animal retreats into its shell, causing 

 the descent of the posterior portion of the ciliated discs, and the 

 parts to which the muscular fibres are attached. I could not 

 make out the position and course of the muscular bundles by 

 which the embryo protrudes the ciliated discs and foot from the 

 shell. The upper and anterior part of the body a little below 

 the base of the foot is attached to the anterior margin of the 

 shell, so that it undergoes little change of position during these 

 movements of retraction and extrusion. The whole structures 

 of the embryo are much more transparent than at an earlier 

 period of its development, no minute cells or nuclei now adhere 

 to the inner surface of the shell, and their number in the other 

 parts is much diminished. The other parts of the embryo ap- 



* The nuclei adhering to the inner surface of the larger of these two cells 

 appeared bigger than the nuclei of which the ovum was originally chiefly 

 composed, and also than those in the other parts of the embryo. 



f This band is more distinctly seen in the embryo of the Dendronotus ar- 

 borescens. 



X This muscular bundle is indistinctly indicated in fig. 19. 



