INVERTEBRATA. 



389 



the zooids appear to be attached is in reality not a distinct 

 structure, but is composed of the basal portions of the 

 successive zooids, for the whole gemmarium consists en- 

 tirely of a series of zooids each budding from its predecessor. 

 Each zooid produces but one bud, and the buds are first 

 on one side and then on the other alternately. 



3. The Perisarc. The whole gemmarium is surrounded 

 by a cuticular secretion of the ectoderm, called the perisarc. 

 This is not in contact with the ectoderm, but separated 

 from it by an interval, except at occasional spots, where 

 the ectoderm cells extend to meet it. The tubular perisarc 

 expands at the end of each branch into a conical cup open 

 at the end, which protects each zooid. This cup is the 

 hydrotheca, and into it the zooid when contracted is with- 

 drawn. Between each hydrotheca and the stem the perisarc 

 is divided by annular constrictions into a number of rings, 

 and there is usually one such ring in the stem itself above 

 the origin of each zooid. In the interior of the hydrotheca 

 near its base the perisarc forms a little annular shelf, on 

 which rests the base of the " hydranth," by which name 

 the enlarged terminal portion of the zooid is distinguished. 

 This base is continued into a narrow stalk, the hydrocope, 

 which passes into the peduncle of the zooid. The interior 

 of the whole organism is occupied by the coelenteric cavity, 

 everywhere continuous. The upright stems arise from a 

 root-like branching system of filaments, which have the 

 same structure as the stems, but are much thinner : these are 

 attached by their whole length to the surface of a sea- weed. 

 The details of structure of the hydroid zooid or hydranth 

 are represented in fig. 198, as seen in a longitudinal section 

 highly magnified. It will be seen that the structure is 

 closely similar to that of the fresh-water Hydra. The 

 ectoderm is thin, and consists of columnar cells and small 

 interstitial cells. In it there are numerous nematocysts, 

 more especially in the tentacles, where they are arranged 

 in annular ridges. The endoderm cells are large and 

 granular ; most of them are furnished with flagella, which 

 are in vibratory motion during life, but the flagella may 

 be withdrawn and pseudopodial processes protruded. 



