62 



INVERTEBRATE ANIMALS. 



sarc, which is furnished with a horny polypary, the whole 

 colony assuming a singularly close resemblance to a plant. In 

 Cordylophora the only fresh-water member of the order we 

 find also a branched composite hydrosoma carrying numerous 

 polypites, and having the ccenosarc defended by a horny sheath 

 (Fig. 15, #, b). In Coryomorpha, finally, we have a type of the 



FIG. 15. a Fragment of Cordylophora lacwtris, slightly enlarged; 5 Fragment of th* 

 same, considerably enlarged, showing a polypite and three gonophores in different 

 stages of growth ; e Portion of Syncoryne /Sarsii, with medusifbrm zooids budding be- 

 tween the tentacles. 



Corynida, in which the hydrosoma consists of no more than a 

 single polypite, and there is no polypary. It is about four 

 inches in length, and is fixed by filamentous roots to the bot- 

 tom of the sea. It consists of a single whitish polypite, striped 

 with pink, and terminating upward in a pear-shaped head, 

 furnished with two sets of tentacles, the shortest of which 

 form a circlet round the mouth. 



As regards the generative process in the Corynida, it may 

 be as well to consider the general phenomena of reproduction 

 as carried on by all the Hydroid zoophytes, the general char- 

 acters of the process being of a most remarkable nature. As 

 has been already explained, the individual in the case of the 

 compound Hydrozoa consists of an aggregation or colony of 

 partially independent beings or zo5ids, produced by gemma- 

 tion or fission from a primordial organism. This is the case 

 in all composite animals, such as sponges, sea-mats, corals, 

 and many others. In many of the compound Uydrozoa, how- 

 ever, the case becomes still further complicated. In many of 

 these organisms, namely, the zo5ids differ very much from one 

 another both in structure and in function. One set of zooids 



