626 



ALFRED C. HADDON. 



To recapitulate — omitting the purely secondary phenomena of 

 the external form and the behaviour of the body-wall — the 

 blastopore closes up and the pedicle of invagination forms 



int 



Fig. 2. — Primary zooecium of Tendra zostericola. The tentacles, though 

 present, are not shown. (After Piepiachoff.) /;;/;., lophophore; int., in- 

 testine ; n. m., retractor muscle; sL, stomach. 



neither the oesophagus nor the intestine. The archenteron is 

 at first solid ; a portion of its substance is prolonged to form 

 the intestine, which subsequently opens to the exterior outside 

 the tentacles. The tentacular sheath and the tentacles are 

 derived from an epiblastic depression, from the floor of which 

 the oesophagus is evaginated, which then fuses with the stomach. 

 The inner face of all these organs is coated with mesoblast. 

 The details of the later development are perfectly normal. 



It is possible that, in some cases, the indifferent character of 

 the cells of the archenteron and the stomodseal invagination, 

 have misled observers into the belief that the embryo has un- 

 dergone histolysis, and that the first zooid of the colony is 

 produced by larval gemmation, for the view of the total 

 formation of a bud (' polypide' of authors) from the endocyst 

 has been so firm that a well-marked involution, such as the 

 stomodffium, would be interpreted as a bud rather than as a 

 portion of the embryo. For myself, I am inclined to believe, 

 with Barrois, that the occurrence of the destruction of the 

 primitive larva is not necessarily universal amongst Polyzoa. 



