VII 



MOLL USGA— 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 E 



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 anteriorlj' on 

 the ventral side, a depression 

 of the ectoderm forms a pit, 

 the stomodaeum (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 endodei-m 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 fj^. 216.-A-G, Stages in the development of Teredo (after 

 have become cylindrical, sink Hatsohek). A, 0, D, E, F, G, from the right side, B in optical 

 in to form the shell gland (F). horizontal section. 1, Ectoderm; 2, macromeres = endoderm 

 The latter secretes the first "^^'^^ • 8, primitive mesoderm cells ; 4, segmentation cavity ; 

 rudiment of the shell in the '' 'to^a^m (cesophagns); 6, ™outh; Y, preoral ciliated 

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

 form of a simple cuticular cephalic plate with tuft; 12, anal invagination, anus; 13, 

 membrane. The endoderm endodermal mid-gut. 

 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 



