the Cape Cephalodiscus. 241 



pigment-spots were distributed over the surface in a uniform 

 manner except at the broader end, where they were more 

 crowded together, forming a dark ring round a white spot. 

 This white spot corresponds in position to a clear spot seen 

 in preserved larvae, and was readily made out in some, but 

 not all, living larvae. It may represent an apical sense- 

 organ. Such embryos rotated on their long axis, and con- 

 tinued to do so when set free from the egg-capsule. . The 

 embryo, partly folded on itself, noted above was evidently a 

 later stage and of the form assumed just before the hatching. 

 The larvae (PI. XI.) were procured from the bottom of the 

 vessel containing the specimens, and only in one instance was 

 a larva seen crawling on the coencecium. They vary some- 

 what in shape. Viewed from above they were ovoid in 

 outline, usually somewhat narrower, but occasionally broad 

 at the posterior end — that is, the end furthest removed from 

 the direction of progression of the larva. The outline was 

 usually regular, but not invariably so. At the posterior 

 extremity there was observed a characteristic indentation or 

 pit, apparently of a superficial nature, sometimes forming an 

 acute angle in its centre, at other times appearing as a mere 

 shallow depression, and in a few cases being apparently 

 absent. 



The size of the living larva was fairly uniform, measuring 

 '57 to "59 mm. in length and about '29 mm. in greatest 

 breadth. It was difficult to secure a satisfactory side-view 

 and exact measurements of the depth of the body, but, by 

 tilting over the crawling larva, it was seen that it was not so 

 flat as it appeared on a surface-view, the depth being about 

 a fifth of its length. 



The pigmentation of the larvae was similar to that of the 

 more advanced embryos, namely, numerous small dark 

 brown dots, rounded, irregular, or elongate in shape, more 

 crowded together at the anterior upper end. That the white 

 spot seen in the embryo was not always visible may have 

 been due to the fact that a large space or clear area now 

 occurs in the anterior region of the body, the posterior third 

 or so being occupied by a more opaque whitish mass, which 

 proved in sections to be a mass of cells heavily laden with 

 yolk. The anterior part of the larva appeared to be quite 

 hollow. This was confirmed by sections, and was very 

 obvious in specimens killed by acetic acid. In preserved 

 specimens this anterior region usually collapsed. 



The behaviour of the larvae was similar in all cases ob- 

 served, except that the rate of progression was more rapid in 

 some. Movement was apparently effected by the action of 



