140 



ZOOLOGY 



SECT. 



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

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

 but certain of its buds— the medusaa — develop gonads, and from 



Fig. 103 — stages in the development of two Zoophytes (A — H, Ziaoinedea, I — M, Euden- 

 driuxn) allied to Obelia ; A — F, stages in segmentation ; G, the planula enclosed in the 

 maternal tissues ; H, the free-swimming planula ; I — M, fixation of the planula and develop- 

 ment of the hydrula. (Fi-om Parker's Biology, after Allman.) 



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

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

 colony, with a sextial generation, or gamohium — 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, inter- 

 stitial cells, muscle-fibres, and nerve-cells. Certain of the inter- 

 stitial cells give rise to characteristic organs of offence — the 

 stinging-capsule.s. The endoderm consists of flagellate or amoeboid 

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

 main forms of zooids, polypes or nutritive zooid.s, which are 

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

 spondence with its locomotive habits, the medusa attains a higher 



