APPENDAGES OF CCELENTEPtATA. 101 



distinct in tlie Mnemidas, owing to tlie presence of lobate processes 

 directed towards the oral polt and most distinct in Cestum, wliere 

 tlie form of the body has become that of a band, from its having 

 grown in the direction of two similar interradii. 



Appendages. 



§ 79. 



I comprise under the head of appendages those processes of the 

 body which are known as tentacles; they are either altogether 

 absent or are only just indicated in the Spongise, but in the 

 Acalephge they are widely distributed, and largely affect the external 

 form of these organisms, in addition to which they are of great 

 physiological importance to its general economy. Most of them 

 are, like the wall of the body, contractile, but there are stiff forms 

 which are not capable of much movement (Trachynemidte) . The 

 tentacles are the seat of a large amount of sensibility, and function 

 as sensory organs ; in many cases they are organs of prehension ; 

 and, finally, they serve as organs of offence by means of the 

 urticating cells which are attached to them. 



The Hydroid- Polyps present the lowest condition; in many 

 divisions of them the tentacles are scattered over the surface of the 

 most anterior portion (or portion lying nearest the oral pole) of the 

 body. In many they may be seen to be arranged more regularly, 

 and in others they form a "circlet of tentacles" (Hydractinia, 

 Eudendrium, Campanularia) . This is generally placed at some 

 distance from the mouth, and gives a higher importance to this 

 part, which appears to be analogous to a head; and, indeed, the 

 tentacular portion of the body (hydranth) of the Hydroida is called 

 a " capitellum." 



The development, in the Tubularia, of a second circlet of 

 tentacles, which directly surrounds the mouth, is correlated with the 

 higher differentiation of the whole body. The outer circlet _ of 

 tentacles is moved to the edge of the hydranth, as this portion 

 becomes flattened out into a disc. Oral and marginal tentacles 

 can then be made out. The latter are greatly developed among the 

 Hydromedusee as well as among the Medusae. 



The marginal tentacles, or marginal filaments, which are 

 generally greatly elongated filamentous appendages of the_ edge of 

 the bell or disc in the Hydromedus^, are always arranged in corre- 

 spondence with the radii of the body. Where interradial tentacles 

 are present, they generally follow the radial ones, even when there 

 is a large number of them. Sometimes they are arranged in tufts 

 (Lizzia), or are branched (Cladonema). In opposition to the increase 

 in the number of tentacles, until it surpasses that of the rays, is the 



