174 ECHIXODERMATA. 



cisely resemble in all the details of their structure those of the 

 Asterias, and their protrusion and retraction are effected in the 

 same manner ; but, in addition to these organs, we find in some 

 genera moveable hooks or spines (Jig. 73, d,) which are likewise 

 retractile, and most probably assist in locomotion. 



(SI 5.) The mouth is a round aperture, as wide as a goose-quill, 

 placed in the centre of a raised ring at the anterior extremity of 

 the body (Jig. 73, a). Around the, oral orifice is placed a circle of 

 tentacula, which are apparently extremely sensible, and serve per- 

 haps not only as instruments of touch, but as prehensile organs 

 used for the capture of prey, or for assisting in deglutition. 

 When the sphincter muscle which closes the mouth contracts, the 

 tentacles are withdrawn, and become no longer visible externally ; 

 in this state, on opening the animal (Jig- 74, &,) they are found 

 to resemble long caeca appended to the commencement of the 

 oesophagus, which have been described by some authors as forming 

 a salivary apparatus. 



The total deficiency of any external skeleton, or calcareous frame- 

 work, precludes, of course, the possibility of the existence of any 

 complex dental apparatus resembling the " lantern of Aristotle ;" 

 the only vestige of the complex teeth of the Echinidse which here 

 remains is a small circle of calcareous pieces, surrounding the opening 

 of the mouth : these plates, from their extreme friability, have been 

 aptly enough likened to laminae of dried paste ; they may indeed 

 in some slight degree be efficient in bruising food taken into the 

 mouth, but it is more probable that they merely form points of 

 insertion to the longitudinal muscles of the body, which, thus fixed 

 around the circumference of the oral orifice, will by their contrac- 

 tion powerfully dilate that aperture for the purpose of taking in 

 nourishment. 



The alimentary canal is of great length, but, like that of the 

 Echinus, presents no stomachal dilatation ; from the mouth, 

 (Jig. 74, a,) in which a bristle is placed, it descends to the anal 

 extremity of the body, where, turning upon itself, it again mounts 

 up towards its commencement, whence turning back again, and 

 forming numerous convolutions, (d, d, rf,) it once more passes 

 backwards, and, becoming restricted near its termination, opens 

 into a large membranous cavity (e) which may be called the 

 cloaca. Throughout the whole of this long course, the alimentary 

 tube is surrounded with a membrane derived from the peritoneal 

 lining of the visceral cavity, which forms delicate mesenteric folds 

 connecting it to the walls of the body, and supporting it through 



