Ova of the Nudibranchiate Mollusca. 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 shellt. 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 heart, 
and the band to be a vessel leading from it, but I obtaimed 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. 20s), from the apex of which a thm membrane 
descends and passes round the body (fig. 20v). 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 
let. 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 ppt). 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. 
¢ This band is more distinctly seen in the embryo of the Dendronotus ar- 
borescens. 
} This muscular bundle is indistinctly indicated in fig. 19. 
