226 ACTINOZOA. 



which occur in the tentacular walls, and partly by 

 the distensive pressure of the fluid forced into 

 the interior of the tentacle, by means of the 

 elastic basal sac. 



An account somewhat different to the above has 

 recently been given by Professor Agassiz^ both of 

 the precise structure of the tentacle itself, and the 

 mode of its connection with the canal-system. 

 " Nearly two-thirds of the length and breadth of 

 the proximate side of the actinal or closed end of 

 the tentacular socket is occupied by an oblong 

 disc, from the mid-length of which the tentacle 

 arises. The distal side of the disc, or that which 

 faces towards the periphery of the body, is convex, 

 with a shallow furrow extending from the base of 

 the tentacle to the actinal end of the disc ; and 

 the proximate side, or that which faces towards 

 the axis of the body, is a plane, immediately 

 beneath whose surface and next to the edge, two 

 chymiferous tubes run parallel ; leaving between 

 them, along the median line, a thick ridge, which 

 is nearly as broad as the diameter of the tubes." 

 — " Only imagine the socket to be removed or 

 reverted, as oftentimes does happen in a great 

 measure, and the whole apparatus mil appear like 

 a peripheric ridge, which, at one point, is drawn 

 out into a slender thread, the tentacle. The base 

 of the tentacle has the form of a high, narrow 

 ridge or keel, more or less plicated or distorted, 

 according to whether the apparatus is extended or 

 retracted ; but we have never seen it projecting 

 beyond the aperture of the socket. At the basal 

 end of the keel it is as broad as the disc from 

 which it arises, but it suddenly narrows to a uniform 

 thickness, which it retains to the other end, where- 



