278 



THE HYDRA 



111 



f§H\ 



sidered in detail, but the general nature of its organization and 

 probable relationships may be indicated. The Physalia is a 

 colony of highly speciahzed individuals having a gas-filled float 



that supports the whole. The 

 tentacles are laden with nemato- 

 cysts which sting so powerfully 

 that their effect on the human 

 hand is like the stinging of 

 many wasps. These capture 

 such prey as small fishes and 

 crustaceans, which are drawn 

 up to the feeding polyps that 

 lie nearer the float. There are 

 also medusa-like units of the 

 colony, which, however, are not 

 set free, but produce the repro- 

 ductive cells. For purposes of 

 comparison, one might say that 

 the siphonophore is like a hy- 

 droid colony that has "gone 

 afloat" and become much 

 speciahzed accordingly. 



Scyphozoa, Actinozoa, and 

 Ctenophora. — Most of the hy- 

 droid jellyfishes are of small size. 

 Gonionemus is larger than the 

 majority, although in a few in- 

 stances such jellyfishes attain 

 considerable dimensions, as in 



Fig. 134.— Physalia floating at the 

 surface of the ocean. 



cr, crest; p, polyp; pn, pneumatophore 

 or float. (After Huxley. From Parker and 



Haswell, "Textbook of Zoology," copyright 1\^q q^SC of MqUOrca, which is 

 1921. by Macmillan & Co., Ltd.. reprinted, ^^^^^^ .^^^^^ .^ diameter. 



by permission.) 



In contrast, the jellyfishes of the 

 type known as Scyphozoa, or scypho-medusse, are mostly forms of 

 large size. In an extreme case, that of the Cyanea nrctica of the 

 New England Coast, there are records of individuals twelve feet in 

 diameter and with tentacles one hundred feet in length. However, 

 the amount of solid material in such an individual would be very 

 small, since jellyfishes have been shown to be composed of as 

 much as ninety-nine per cent water. The genera Dactylometra 

 and Aurelia are also represented upon the Atlantic Coast. In 



