6 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 perradial canals lie 



/" "-^ "_/^ # in the first four radii (Fig. 7) 



• / Y'/"^^ jK" which are accentuated in the 



/ ^^^^^^^__^^ ''" development of the medu- 



-cfcir *:_ii -<~r' "^ "m" soid ; other four radiating 



\ ^_^/^ \% canals may be similarly 



\\ >m ^ <r ^ J formed between these, which 



^~^^- -^ ' with them divide the um- 



K,G Tij^.-Part (.fa soction of ^im7.v, showing brella iiito eight coual parts ; 



e, amoeboid cells in the mesoi^loHa ; e/, eiiiloderin of i • ?• j 



gastral lamella ; fn, findo<lerm lininj; gastric cavity, they are termed inten'ailUll . 

 (From Lankester, after Hertwi^')- \ r.,.4.u„ „.. r • -Ui. !• 



^ 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. 



AVhen medusoids are attached to a hydroid colony, the perisarc 

 in some cases expands into a (jonothcca 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 thk 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-muscular 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 exhil)it striations, are attached to the mesogloea, and 

 the movements of the body arc largely eftected 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"^). The possession of a stiff sensory filament, the i)alpocil, 

 characterises the sense cells. Other cells, }iiovided with a similar 

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

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

 defence. This consists (Figs. 8*^, 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 difierent appearance ; the filament, barbs, and 



