COiLENTERATA I HYDROZOA. 



97 



resemble true polypites in form, but differ from them in being 

 usually devoid of a mouth, and in having shorter tentacles. 

 They are, in truth, atrophied or undeveloped polypites. 



As regards the development of the Corynida, the embryo is 

 very generally, though not always, ciliated at first, and becomes 

 developed into a hydra-form polypite, which fixes itself to 

 some foreign body, and then (if not belonging to one of the 

 simple forms) proceeds to produce by gemmation the com- 

 posite adult. In one species the embryo leaves the gono- 

 phore as a free and locomotive polypite, and in another form it is. 

 non-ciliated and amoeboid. The development of the Corynida 

 (as well as that of the Sertularida and Lucernarida) obeys the 

 general law, that the new polypites are developed at, or near, 

 the distal end of the hydrosoma ; so that the distal polypites 

 are the youngest, the reverse of this obtaining amongst the 

 oceanic Hydrozoa. 



The subject of the reproduction of 

 the Corynida having been treated at 

 some length, so as to apply to the re- 

 maining Hydroida, we shall now give 

 a brief description of the leading types 

 of structure exhibited by the order. 



Eudendrium, a genus of the Cory- 

 nida^ which is not uncommonly 

 found attached to submarine objects, 

 usually in tolerably deep water, may 

 be taken as a good example of the 

 fixed and composite division of the 

 order. The hydrosoma consists of 

 numerous polypites, united by a 

 coenosarc, which is more or less 

 branched, and is defended by a 

 horny tubular polypary. The poly- 

 pites are borne at the ends of the 

 branches and branchlets, and are 

 not contained in "hydrothecae," the 

 polypary ending abruptly at their 

 bases. The polypites are non- 

 retractile, of a reddish colour, and 

 provided with about twenty ten- 

 tacles, arranged round the mouth 

 in a single row. Tubularia (fig. 

 22) is very similar to Eudendrium, 

 but the hydrosoma is either undivided or is very slightly 

 branched. The hydrosoma consists of clustered homy 



G 



Fig. 22 Corynida. Fragment of 

 Tubularia indivisa, natural 

 size. 



