428 ZOOLOGY skct. 



vessels. It is made up of strands of a kind of gelatinous connec- 

 tive tissue, with many leucocytes, permeated in a very irregular 

 way by minute lacunas without definite walls. The great 

 development of the gastric and intestinal branches of this system 

 in some (Echinoids, Holothuroids), lends support to the view that 

 its main functions are connected with the absorption and 

 distribution of nourishment. 



The axial organ (genital stolon) of the Echinodermata is 

 closely connected both with the perihsemal and hiemal systems. 

 Its general structure and relations in the Asteroidea have already 

 been described (p. 384). 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 Cucu- 

 maria. The Cuvierian organs, which occur only in a limited 

 number of Holothurians, correspond to undivided basal branches 

 of the respiratory trees : they are defensive organs, the animal 

 when attacked throwing out numbers of these long filaments, 

 which are very viscid and 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 



