150 



METAZOAN PHYLA 



viduals in the colony. In the Portuguese man-of-war (Fig. 68), for 

 example, there are polyps which are nutritive, others which are sensory, 

 others which contain batteries of nematocysts as weapons of offense and 

 defense, still others which contain male gonads, and finally some which 



give rise to egg-producing medu- 

 sae. There remain to be added 

 to this enumeration polyps which 

 unite in the production of a gas 

 bag that serves to float the organ- 

 ism at the surface of the sea. 

 This kind of polymorphism, where 

 the unlike individuals are united 

 in a single organism, is very rare. 

 Polymorphism, however, is very 

 general in the animal kingdom and 

 may or may not be accompanied 

 by division of labor. 



178. Metabolism. — In most 

 coelenterates ingestion of food 

 occurs by means of the tentacles, 

 which secure the food and pass it 

 into the mouth. As a hungry 

 jellyfish is carried along through 

 the water by a current, aided in 

 some cases by pulsations of the 

 bell, its tentacles trail below and 

 behind forming a net in which the 

 prey is entrapped. These tenta- 

 cles may be spun out till they 

 resemble exceedingly fine threads ; 

 they are not strong, but, fur- 

 FiG. 68. — A Portuguese man-of-war, Phy- nished as they are with batteries 

 :;!ri'ZSr X «^'Th?f»tt;f t"?.: of nematocysts which soon para- 



pable of extension to a length of over 40 feet, lyzC the struggling victim, they 

 and bear thousands of minute nematocysts. ^^^^^ ^^^^^ ^^^-^^ purpOSe. The 



larger jellyfishes capture many animals of considerable size, including even 

 fish. In a similar manner sea anemones soon quiet the luckless animal 

 which runs or falls upon the expanded tentacles, after which it is passed 

 from one group of tentacles to another until it is put into the mouth. 

 The smaller jellyfishes cannot sting severely enough to be noticed by a 

 human being, but the larger ones may cause a marked effect, the sensa- 

 tion being similar to that following the sting of nettles. 



The steps in metabolism in all coelenterates are similar to those 

 described for hydra. 



