394 ZOOLOGY SECT. 



The system of vessels and sinuses to which the designation 

 blood-vascular system is applied are specialised extensions of 

 the ccelome, from the main cavity of which they are not completely 

 separated off. Their walls are for the most part lined by a ciliated 

 epithelium by means of which the movement of the contained 

 fluid which does not differ from that in the ccelome, is brought 

 y about. There is never any contractile part acting as a heart. The 

 general disposition of the parts of this system in the various classes 

 has already been referred to in the descriptions of the examples. 

 The arrangement in the Ophiuroidea resembles that described in 

 the Starfish. In the Holothuroidea and Crinoidea the axial sinus 

 and aboral ring-vessel, present in the other three classes, are 

 absent, and there are large intestinal vessels accompanying the 

 enteric canal. 



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 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 c?3ecum, 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. 



In the Crinoidea the alimentary canal is simply a coiled tube 

 with both mouth and anal opening on the same (ventral) 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 m the other classes. The stomach is divided, as already 

 described in the account of tlie examples, into two portions, the 

 cardiac and the pyloric, the former giving off five large rounded 



