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COMPARATIVE ANATOMY. 



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elements ; which are found in all the Acalephas, although they 

 are not confined to them. They are firm capsules (Fig. 35, B), 



which are formed in the protoplasm of the 

 1 A z cell, and in which an elastic, spirally- coiled 



§A thread (A) is found ; this is generally emitted 



in the form of a stiff body, when the cap- 

 siile is touched. These " stinging organs " 

 are sometimes solitary, sometimes in groups, 

 and at times they are very regularly arranged. 

 They often become greatly complicated, as 

 in the stinging knots of the Siphonophora, 

 where they are often arranged in spiral 

 bands. These ' ' stinging batteries " develop 

 on the surface, but are often provided with 

 a special investment formed by a fold of 

 the integument. 



Although these structures are scattered 

 over the whole surface of the body, and are 

 not absent even from the endoderm and its 

 products, yet many parts of the body are 

 specially characterised by them ; above all, 

 the tentacles, or other processes of the body. 

 The urticating capsules vary greatly in form, 

 as does the filament in structure ; and these 

 differences are characteristic of the different 

 divisions. 



Further, the ectoderm has a secreting 

 activity, by which tests, which more or 

 less invest the body, are formed. They are 

 very common among the Hydroid-Polyps, 

 where they are formed of a firm substance 

 allied to chitin, and are often provided with 

 various sculpturiugs, flutings, spines, ridges, 

 and so on. These tubular tests are especially 

 found among the colonial Hydroid-Polyps ; 

 they are sometimes limited to the fixed 

 portion of the common stock (Hydractinia), 

 sometimes continued on to the branches 

 of the stock (Tubularia, Eudendrium, Pennaria), and sometimes 

 they are found even on the separate persons (Campanularia, 

 Sertularia). This provides the soft polyp-stock with an organ of 

 support, by which it is enabled to raise itself from the ground, as 

 well as to attach itself. 



Fig. 35. Different forms of 

 urticating capsules, a Cap- 

 sule of Corynactis ; 1 With 

 the filament spirally coiled ; 

 2 Extended. B C Capsules 

 of Siphonophora : the fila- 

 ments extended and partly 

 provided with hooks. D Ur- 

 ticating cells of Medusas ; 

 filaments still rolled up ; in 

 one not yet differentiated, 

 and the nucleus of the cell 

 still visible. 



