162 



ZOOLOGY 



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 Siphonophore : the thick line represents endoderm ; the space ex- 

 ternal to it, ectoderm ; the internal space, the enteric cavity, cce. coenosarc ; dz. dactylozooid ; 

 hph. iiydrophyllium ; md. sporosac ; net, net', nectocalyces ; ntc. battery of nernatocysts ; 

 p. polype ; pn. pneumatophore ; 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 



