162 



ZOOLOGY 



SECT. 



The egg of Halistemma gives rise to a ciliated planula re- 

 sembling that of the other Hydrozoa. At one pole the ectoderm 

 becomes invaginated to form the float (Fig. 121, ep], the opposite 

 extremity is gradually converted into the first polype (po), and 



net 



FIG. 120. Diagram of a Siphpnophore : the thick line represents endoderm ; the space ex- 

 ternal to it, ectoderm ; the internal space, the enteric cavity. c<x. ccenosnrc ; Jz. dact3 T lozooid ; 

 kph. hydrophyllium ; m<l. sporosae ; net, net', nectocalyces ; ntc. battery of nernatocysts ; 

 p. polype ; p/i. pneuinatuphore ; /. tentacle. (After Clans.) 



a bud appears on one side which becomes the first tentacle (t). 

 By gradual elongation, and the formation of new zooids as lateral 

 buds, the adult form is produced ; the various zooids are all 

 formed between the first polype and the float, so that the two 



