150 



ZOOLOGY 



SECT. 



dividing these structures into two equal halves : the same applies 

 to the polype and feelers with their single basal tentacle. When 

 first formed the various zooids are all on one side of the stem, but 



fan 



TICt 



FIG. 107. Diagram of a Siphonophore : the thick line represents endoderm ; the space ex- 

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

 hjrfi. hydrophyllium ; net. net', nectocalyces ; ntc. battery of ncmatocysts ; p. polype ; pn. 

 pneumatophore ; t tentacle. (After Glaus.) 



the latter becomes spirally twisted during growth, and so causes 

 them to arise irregularly. 



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. 108, ep), the opposite 

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



