204 ECHINODEEMATA. 



(533.) The mouth is a round aperture, as wide as the bore of a goose- 

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

 the body (fig. 100, a). Around the oral orifice is placed a circle of ten- 

 tacula, which are apparently extremely sensitive, and serve perhaps 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 muscle that closes the mouth contracts, the tentacles are withdrawn, 

 and become no longer visible externally ; in this state, on opening the 

 animal (fig. 101, 6), they are found to resemble long caeca appended to 

 the commencement of the oesophagus, and have been described by some 

 authors as forming a salivary apparatus. 



(534.) The total deficiency of an external skeleton or calcareous 

 framework 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 Echinidas 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 for the longitudinal 

 muscles of the body, which, thus fixed around the circumference of the 

 oral orifice, will by their contraction powerfully dilate that aperture for 

 the purpose of taking in nourishment. 



(535.) The alimentary canal is of great length, but, like that of the 

 Echinus, presents no stomachal dilatation ; from the mouth (fig. 101, 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 d), 

 it once more passes backwards, and becoming constricted near its termi- 

 nation, opens into a large membranous cavity (e) that 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 its entire length. The 

 whole intestine is generally found distended with sand, wherein may be 

 detected the debris of corals, algae, fuci, and other marine substances. 



(536.) In the structure of the respiratory apparatus, the Holothurida3 

 differ materially from the rest of the Echinodermata, and, in fact, from 

 all other animals. In the Holothuria, the aeration of the circulating 

 fluid is provided for by allowing the surrounding element freely to enter 

 into the internal parts of the creature ; but instead of bathing the sur- 

 faces of the viscera, the water is confined in a peculiar system of rami- 

 fying canals, forming a structure of great beauty and, frqjp its singu- 

 larity, extremely interesting in a physiological point of view. We have 

 seen that the intestinal canal terminates in a membranous receptacle or 



