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. 



thick cap of ectoderm cells (micromeres) 

 A 



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), 



closely covering these, and 



two symmetrical primitive 



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 



Fici. 21G.— A-G, Stages in the development of Teredo (after 

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

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

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

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

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

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

 endodermal mid-gut. 



