THE COELENTERATES AND CTENOPHORES 



93 



suppression is found in members of the genus Hydra, wherein 

 the gonads occur in the ectoderm with practically no remnant 

 of spadix or other medusoid remains (Fig. 52 D). 



Suppression of the Hydroid. — In contrast with the gonophore 

 reduction discussed above stands the suppression of the hydroid 

 generation characteristic of representatives of the order Trachy- 

 linae. In these, development of the medusa proceeds directly 

 from the larva derived from the fertilized egg without the inter- 

 vention of the polyp generation. 



Polymorphism in Hydrozoa. — The original plan of two body 

 forms alternating in the life cycle of the Hydrozoa has become 

 much modified in members of one order called the siphonophores 

 (Siphonophora). These are free-float- 

 ing or swimming colonial forms in 

 which the parts have become so 

 interdependent and so highly special- 

 ized for limited functions that it 

 becomes difficult to distinguish whether 

 the individuals of the colony are 

 hydroid or medusoid." A continuous 

 tube of the digestive system connects 

 all the individuals of a colony and each 

 performs but a limited service for the 

 entire colony. One individual consists 

 of only an air-filled bladder or pneu- 

 matophore (Fig. 53 A), which regulates 

 the position of the colony at or 

 beneath the surface of the ocean. 

 Nectocalyces, or swimming bells (B, C), 



Fig. 



Diagram of a 



frequently occur just below the float siphonophore colony (Phy- 

 1 ■ 1 ,^ 1 .,1 sophorida). A, pneumato- 



and provide the colony with a means p^ore; b, c, swimming bells; 

 of locomotion. These are lacking in -D. protective zooid; E, sporo- 



P ,, . -Ill • 1 sac; F, G, dactylozooids; H, 



one of the most widely known sipho- fg^jing polyp; I, nettling 

 nophores, the Portuguese man-of-war cells. {After Claus). 

 (Physalia), whose greatly enlarged 



pneumatophore acts as a sail. The remainder of the colony 

 consists of feeding zooids H, dactylozooids as sensory organs F 

 and G, sporosacs E, bracts or organs of defence D, and batteries 

 of nettling cells /. The individuals are so completely interde- 

 pendent and are so highly specialized for carrying on limited 

 functions for the colony as a whole that the colony is frequently 



