Jelly fishes. 35 



only differ in the details of their organisation and 

 arrangement. In many of the marine forms, the 

 hydra-like animals are grouped in clusters or colonies 

 on a branched common axis or stalk (fig. 20). In 

 each of these colonies every hydra-like organism is 

 called a polypite, and the common stalk of the colony 

 is called the ccenosarc. 



These colonies we find on the sea-shore as branch- 

 ing tree-like growths on sea-weeds, often exceeding six 

 inches in length, and some of them are known as * sea- 

 firs.' Each group begins its existence as a single poly- 

 pite, rooted by that extremity of its body which is oppo- 

 site to the mouth. From this root a stalk of ccenosarc 

 grows upwards, and on this stalk new polypites form, 

 each like a hydra in structure. New polypites arise as 

 outgrowths from, and are structurally continuous with 

 the common stem of the colony ; and so the stomach 

 of each is continuous with the tubular centre of the 

 stalk. On this account there is a community of nutri- 

 tion in the colony, a few actively-feeding polypites 

 being able to make up for the laziness of others, which 

 participate in the nourishment taken in by their more 

 active neighbours. 



Medusoids. In these colonies some polypites are 

 set apart for the production of eggs for the multiplica- 

 tion of individuals (fig. 21, c, D). Such polypites are 

 much altered in form, and they frequently become 

 detached from the stem and float about freely often 

 as little jellyfishes about the size of a pea ; they are 

 called medusoids. With a fine muslin surface-net in 

 any of our seas, many forms of these little creatures 

 may be captured, especially in summer or autumn; 

 D 2 



