162 



ZOOLOGY 



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

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

 becomes invaginated to form the float (Fig. 121, cjj), the opposite 

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



nc2- 



Fig. 120.— Diagram of a Siphonophore : the thick line represents endoderm ; the space ex- 

 ternal to it, ectodemi ; the internal space, the enteric cavity, co?. co^nosnrc ; ih. dactylozooid ; 

 hpli. hydruphyllium ; imi. sporosac ; vet, net', nectocalyces ; ntc, battery of nematocysts ; 

 'p. polype ; pn, pneuniutopliuru ; t. tentacle. (After Glaus.) 



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 



