ix PHYLUM ECHINODERMATA 419 



ment of the gastric and intestinal branches of this system in some 

 (Echinoids, Holothuroids) lends support to the view that its main 

 functions may be connected with the absorption and distribution 

 of nourishment. 



The axial organ (genital stolon) of the Echinodermata is 

 closely connected both with the perihsemal and haemal systems. 

 Its general structure and relations in the Asteroidea have already 

 been described (p. 378). In the Ophiuroidea there is a close 

 correspondence with the Asteroidea, the chief differences being 

 such as are involved in the change in the position of the madreporite 

 from the aboral to the oral surface, and the resulting change in the 

 direction of the madreporic canal and associated axial sinus and 

 axial organ. In the Echinoidea the essentials are the same ; but 

 the axial organ has grown round the axial sinus so as to enclose it 

 completely. 



The enteric canal varies in the five classes more than any 

 of the other systems of organs. It is a simple tube in the Holo- 

 thurians and Echinoids, passing spirally through the body from the 

 mouth at the oral pole to the anus at the opposite pole. In most 

 of the latter group a complex masticatory apparatus with five 

 teeth the so-called " lantern of Aristotle " is situated at its 

 anterior extremity ; the corresponding region in the Holothurians 

 is surrounded by a circlet of ossicles, which protect the nervous 

 and vascular rings and into which the longitudinal muscles of the 

 body-wall are inserted. 



In the Echinoidea there is a tubular caecum, the siphon, con- 

 nected with the intestine. In the Holothurians the so-called 

 " respiratory trees " (absent in the Elasipoda and the Apoda) are 

 branched appendages usually two in number, sometimes single 

 of the cloaca or posterior wider portion of the intestine, and 

 the " Cuvierian organs " are simple filiform glandular tubes, also 

 connected with the cloaca. 



The functions of the siphon and of the respiratory trees have 

 already been referred to in the accounts of Echinus and Cucumaria. 

 The Cuvierian organs, which occur only in a limited number of 

 Holothurians, correspond to undivided basal branches of the respira- 

 tory trees : they are defensive organs, the animal when attacked 

 throwing out numbers of these filaments, the secretion of which is 

 very viscid and assumes the character of slender threads which 

 may have the effect of entangling and hampering the assailant ; 

 but they may also have an excretory function. 



In the Crinoidea the alimentary canal is simply a coiled tube 

 with both mouth and anal opening on the same (actinal) surface 

 of the body. In the Ophiuroids the central mouth leads into a 

 simple sac giving off short diverticula, and there is no anal 

 aperture. In the Asteroidea the alimentary canal is more complex 

 than in the other classes. The stomach is divided, as already 



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