162 



OHAS. B. WILSON. 



their derivatives. The entire surface of the pilidium is 

 covered with cilia^ those along the edge of the helmet being 

 thicker and stouter than elsewhere_, since the pre-oral ring of 

 the gastrula is continued around the edge of the lappets. 

 By a contraction of the muscles with which the lappets are 

 soon supplied a peculiar flapping movement is produced, very 

 similar to that of a pecten^s shell when swimming, and this 

 seems to be of assistance in locomotion. The lappets are 

 also quite sensitive, though they are always carried behind 

 the larva when swimming, and hence cannot be of as much 

 service as the flagella. 



The larva has now become perfectly transparent, so that 

 the details of internal development can be easily watched. 



The Primitive Intestine. — At first no differentiation is 

 possible in the invagination which forms the primitive 

 intestine. It is composed throughout of a single layer of 

 cells which are longer and narrower than those of the 

 ectoderm, and which bear cilia upon their internal faces. 

 These cilia are very delicate, ajid are manifestly the result of 

 filose action. As soon as the embryo elongates, however, 

 the invagination bends over toward one end, the future 

 posterior end of the embryo (fig. 10). This flexed end 

 approaches close to the wall of the pilidium, enlarges, and 

 assumes a spherical shape. 



A slight constriction appears at the point of flexure, and 

 we can now distinguish an oesophageal portion which 

 extends from the mouth to the point of constriction, and a 

 stomach portion, the enlarged inner end. Histological 

 examination reveals a clear distinction in structure also 

 between the two (cf. p. 188). 



As the stomach enlarges the aboral surface posterior to 

 the lappets becomes larger than the corresponding anterior 

 portion. 



Consequently it rounds over somewhat where it joins the 

 oral surface, and the latter sinks in as a shallow saucer- 

 shaped depression, approaching closely to the wall of the 

 stomach. But the two never meet as in the trochophore larva, 



