148 



METAZOAN PHYLA 



viduals in the colony. In the Portuguese man-of-war (Fig. 64), 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. 



176. Metabolism. — In all coe- 

 lenterates 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 tentacles may 

 be spun out till they resemble 

 exceedingly fine threads; they are 

 not strong, but, furnished as they 

 are with batteries of nematocysts 

 which soon paralyze the struggling 

 victim, they serve well their 

 purpose. The larger jelly fishes 

 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 sensation being similar to that following stinging by nettles. 

 The steps in metabolism in all coelenterates are similar to those 

 described for hydra. 



Fig. 64. — A Portuguese man-of-war, Phy- 

 salia pelagica Bosc. {From. Packard, "Zoology," 

 after Agassiz.) X ^. The tentacles are ca- 

 pable of extension to a length of over 40 feet, 

 and bear thousands of minute polyps. 



