REPRODUCTION 



175 



Extreme Polymorphism. — Remarkable examples of colony formation 

 and metagenesis, accompanied by division of labor among the types of 

 individuals that reproduce asexually, occur among' the marine animals 

 known as siphonophores, which have a structural similarity to Hydra. 

 The siphonophores are free-swimming colonies of varying complexity. 

 Each colony (Fig. 145) consists of a common tube of coenosarc which 

 bears at one end a pneumatophore 



■ z i 



D 



or float and along its length zooids 

 of various forms. The float is 

 the expanded end of the coeno- 

 sarcal tube. It generally con- 

 tains gas and serves to support 

 the colony which hangs freely in 

 the water. Near the float is a 

 group of swimming bells {nedo- 

 calyces) which resemble medusae 

 and whose function it is to propel 

 the colony through the water by 

 their alternate contraction and 

 expansion. At intervals l:)elow 

 the swimming bells occur bracts, 

 or covering scales ; feeding polyps 

 which ingest the prey and digest 

 it for the entire colony; sensory 

 polyps which in some species at 

 least also serve as digestive or- 

 gans; tentacles (defensive and 

 offensive individuals) provided 

 with nematocysts (page 72) ; and 

 gonopJiores (reproductive zooids) 

 with or without bells. A first 

 examination of a siphonophore 

 might lead to the conclusion that 

 it is a complex individual with 

 half a dozen kinds of organs. By 

 a careful study of selected forms, 

 however, and by means of a 

 comparison of these with such forms as Obelia, it may be determined 

 that most of the structures which in a siphonophore resemble and 

 function as organs are really much modified individuals, either polyp 

 or medusa (Figs. 143, 144). In certain species the bracts contain 

 remains of radial canals which are characteristic of medusae. The 

 bracts, swimming bells, and gonophores are constructed on a medusoid 



/ 



Fig. 146. — Physalia, the Portuguese man- 

 of-war, drawn from live animal floating on the 

 surface of the sea. cr, crest; p, polyp; pn, 

 pneumatophore; t, tentacle. {From Parker 

 and Haswell, " Textbook of Zoology,'^ after 

 Huxley.) 



