140 



ZOOLOGY 



SECT. 



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 



FIG. 103 Stages in the development of two Zoophytes (A H, Laomedea I M, Euden- 

 drium ) 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. (From Parker's Biology, after Allruan.) 



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, inter- 

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

 stitial 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 arc 

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

 spondence with its locomotive habits, the medusa attains a higher 



