IV PHYLUM CCELENTERATA 141 



weed, &c., fixes itself by one end (K), and becomes converted into 

 a liydrula or simple polype (L, M), having a disc of attachment at 

 its proximal end, and at its distal end a manubrium and circlet of 

 tentacles. Soon the hydrula sends out lateral buds, and, by a 

 frequent repetition of this process, becomes converted into the 

 complex Obelia-colony with which we started. 



This remarkable life-history furnishes the first example we have 

 yet met with among the Metazoa of alternation of generations, or 

 metagenesis (see p. 41). The Obelia-colony is sexless, having no 

 gonads, and developing only by the asexual process of budding ; 

 but certain of its buds the medusae develop gonads, and from 

 their impregnated eggs new Obelia-colonies arise. We thus have 

 an alternation of an asexual generation, or agamobium the Obelia- 

 colony, with a sexual generation, or gamobium the medusa. 



2. GENERAL STRUCTURE AND CLASSIFICATION. 



The Hydrozoa may be defined as multicellular animals in which 

 the cells are arranged in two layers, ectoderm and endoderm, 

 separated by a gelatinous, non-cellular mesogloea, and enclosing 

 a continuous digestive cavity which communicates directly with 

 the exterior by a single aperture the mouth and is lined through- 

 out by endoderm. The ectoderm consists of epithelial cells, 

 interstitial cells, muscle-fibres, and nerve-cells. Certain of the 

 interstitial cells give rise to characteristic organs of offence the 

 stinging-capsules. The endoderm consists of flagellate or amoeboid 

 cells, gland-cells, and sometimes muscle-fibres. There are two 

 main forms of zooids, polypes or nutritive zooids, which are 

 usually sexless, and medusse or reproductive zooids. In corre- 

 spondence with its locomotive habits, the medusa attains a higher 

 degree of organisation than the polype, having more perfect 

 muscular and nervous systems, distinct sense-organs, and a diges- 

 tive cavity differentiated into central and peripheral portions, the 

 latter taking the form of radial and circular canals. The repro- 

 ductive products are discharged externally, and are very commonly, 

 though not always, of ectodermal origin. 



Many Hydrozoa agree with Obelia in exhibiting alternation of 

 generations, the asexual generation being represented by a fixed, 

 more or less branched, hydroid colony, the sexual generation by a 

 free-swimming medusa. In other forms there are no free medusee, 

 but the hydroid colony produces fixed reproductive zooids. In 

 others, again, there is no hydroid stage, the organism existing only 

 in the medusa-form. Then, while in most instances the only 

 skeleton or supporting structure is the horny perisarc, there are 

 some forms in which the coenosarc secretes a skeleton of calcium 

 carbonate, forming a massive stony structure or coral. Lastly, 

 there are colonial forms which, instead of remaining fixed, swim 



