PHYLUM CCELEKTERATA 



163 



the latter becomes spirally twisted during growth, and so causes 

 them to arise irregularly. 



The egg of HaHstemma gives rise to a ciliated planula resem- 

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

 becomes invaginated to form the float (Fig. 122, ep), the opposite 

 extremity is gradually 

 converted into the first 

 polype (po), and 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 

 become further and 

 further apart, being 

 always situated at the 

 distal and proximal 

 ends of the colony 

 respectively. 



In an allied form 

 (Agalma) the first structure 

 to appear in the embryo is 

 not the float, but the first 

 bract, which grows con- 

 siderably and envelops the 

 growing embryo in much 

 the same way as the um- 

 brella of a medusa envelops 

 the manubrium. On this 

 and other grounds some 

 zoologists look upon the 

 Siphonophore-colony as a 

 medusa the manubrium of 

 which has extended im- 

 mensely and produced 

 lateral buds after the 

 manner of some Antho- 

 medusse (Fig. 106, 7 a). On 

 this theory the entire crenosarc is an extended manubrium and the first or 

 primary bract is the umbrella. But frequently as in Halistemma a 

 primary bract is not formed, and when present there appears to be no reason 

 against regarding it as a lateral bud of the axis, of quite the same nature 

 as the remaining zooids. 



In the well-known " Portuguese man-of-war " (Physalia) there 

 is a great increase in proportional size of the float and a corre- 



M 2 



. 123. Physalia : the living animal floating on the 

 surface of the sea. cr. crest ; p. polype ; pn. pneu- 

 matophore ; t. tentacle. (After Huxley.) 



