THE HYDROMEDUSAE 



canals, circular canal, and sometimes tentacular canals ; the 

 endoderm, in addition to forming the lining of these cavities, 

 forms the endoderm lamella, and sometimes a solid tentacular core. 



The permdial canals lie 

 in the first four radii (Fig. 7) 

 which are accentuated in the 

 development of the niedu- 

 soid ; other four radiating 

 canals may be similarly 

 formed between these, which 

 with them divide the uni- 



Fio. 7**. Part of a section of Aurelia, showing brella into eiVht Pnnn.1 nart* 

 r , amoeboid cells in the mesogloea ; ri, endoderm of ' 

 gastral lamella ; en, endoderm lining gastric cavity, they are termed intenrodial. 



A further set of eight radi- 



ating canals is sometimes developed between perradial and inter- 

 radial canals, and is termed adradial. 



The exumbral mesogloea is generally greatly thickened and 

 adds firmness to the bell. 



When medusoids are attached to a hydroid colony, the perisarc 

 in some cases expands into a gonotheca for their protection (Fig. 17) ; 

 in other cases it is absent (Fig. 16). 



Medusoids are formed either as buds from hydroids or from 

 hydroid coenosarc, or as buds from other medusoids, or directly 

 from the fertilised ovum. They are typically free swimming and 

 fertile, and are often incapable of taking food. 



HISTOLOGY OF THE HYDROID (Figs. 8 to 10) (Jickeli, 17; 

 v. Lendenfeld, 18). The ectoderm is generally composed of a 

 single layer of cells, and includes several varieties of cell forms. 

 Of these the most prominent are the large epithelio-muscidar cells, 

 the inner ends of which give off contractile fibres in a direction 

 parallel to the long axis of the body; these fibres, which fre- 

 quently exhibit striations, are attached to the mesogloea, and 

 the movements of the body are largely effected by their means. 

 In some cases a gradual diminution can be traced in the size of 

 the cell body, and a corresponding increase in the size of the 

 muscular fibre ; this leads to a deep-lying muscle cell, no longer 

 epithelial, comparable to the smooth muscle cell of Triploblastica 

 (Fig. 8, 1-3 ). The possession of a stiff sensory filament, the palpocil, 

 characterises the sense cells. Other cells, provided with a similar 

 filament, the cnidocil, are termed cnidoblastSj and secrete in the 

 interior of the cell body the nematocyst, a weapon of offence and 

 defence. This consists (Figs. 8 6 , 9) of a vesicle, often with double 

 walls, filled with fluid, the neck of which is barbed and then drawn 

 out into a long and extremely fine tubular filament, at the tip of 

 which the tube probably opens to the exterior. When in the cell, 

 the nematocyst has a different appearance ; the filament, barbs, and 



