MOLL USC A ONTOGENY 



259 



cells sink below the surface, and form on each side an ectodermal cell mass, which 

 becomes detached from the rest of the ectoderm, sinks into the mesoderm of the 

 foot, and fuses with the similar mass on the other side to form the pedal ganglion. 



D. Lamellibranchia. 



1. Development of Teredo (Figs. 216 and 217). Segmentation is here total and 

 unequal. The gastrula, formed by epibole (Fig. 216 A, B) consists of (1) two 

 large endoderm cells (macromeres), a thick cap of ectoderm cells (micromeres) 

 closely covering these, and 

 two symmetrical primitive A 



mesoderm cells of medium 

 size at the posterior edge of 

 the blastopore. The blasto- 

 pore closes from behind for- 

 ward, the ectoderm cells by 

 continual division growing 

 entirely round the endoderm 

 cells ; during this process the 

 two mesoderm cells become 

 covered by the ectoderm and 

 come to lie between the latter 

 and the endoderm (Fig. 216 

 C). Somewhat anteriorly on 

 the ventral side, a depression 

 of the ectoderm forms a pit, 

 the stomodseum (D). The 

 ectoderm separates off from 

 the two - celled mesoderm, 

 thus giving rise to a seg- 

 mentation cavity, or primary 

 body cavity. A double 

 preoral ciliated band is 

 formed (D, E). The two 

 large endoderm cells, by 

 fission, produce other smaller 

 cells. Cilia appear over the 

 whole surface of the germ, 

 with the exception of the 

 posterior dorsal surface, where 

 the ectoderm cells, which 

 have become cylindrical, sink 

 in to form the shell gland (F). 

 The latter secretes the first 

 rudiment of the shell in the 

 form of a simple cuticular 

 membrane. The endoderm 

 cells begin to collect to form 

 the intestinal wall. After the formation of the first rudiment of the shell, the shell 

 gland flattens and spreads out ; its edge can still be found as a ridge running under 

 the edge of the shell. The endoderm now forms a large globular hollow mid-gut, 

 into which the oesophagus breaks through. Each of the primitive mesoderm cells 



FIG. 216. A-G, Stages in the development of Teredo (after 

 Hatschek). A, C, D, E, F, G, from the right side, B in optical 

 horizontal section. 1, Ectoderm; 2, macromeres = endoderm 

 cells ; 3, primitive mesoderm cells ; 4, segmentation cavity ; 

 5, stomodseum (oesophagus); 6, mouth; 7, preoral ciliated 

 band ; 8, shell gland ; 9, shell ; 10, larval muscle cells ; 11, 

 cephalic plate with tuft ; 12, anal invagination, anus ; 13, 

 endodermal mid-gut. 



