ECHINODERMATA : ECHINOIDEA. 199 



"regular." Sometimes, however, the anal aperture is with- 

 out the apical disc, and is removed to some distance from 

 the genital plates, when the test is said to be "irregular." 

 In this last case, the anus, instead of being apical, is marginal 

 or submarginal. The convolutions of the alimentary canal 

 are attached to the interior of the test by a delicate mesentery ; 

 the surface of which, as well as that of the lining-membrane 

 of the shell, is richly ciliated, and subserves the purposes of 

 respiration. 



The proper blood-vascular system (fig. 96 A, b) consists of 

 a central fusiform, contractile vesicle, or heart. This gives off 

 one vessel which forms a ring round the intestine near the 

 anus, and another which passes downwards, and forms a circle 

 round the gullet, above the " circular canal " of the ambulacral 

 system. From the anal vessel proceed five arterial branches, 

 which run along the ambulacral spaces, and return their blood 

 by five branches, which run alongside of them in an opposite 

 direction. This system of vessels is not always present, and 

 its true nature is doubtful. High authorities regard it as rather 

 comparable to the " pseudohagmal " system of the Annelides, 

 than to the blood-system of the higher animals ; while eminent 

 observers maintain that the so-called heart is really of a gland- 

 ular nature. 



The nervous system consists of a ganglionated circular cord, 

 which surrounds the gullet below, or superficial to, the " cir- 

 cular canal " of the ambulacral system, and which sends five 

 branches along the ambulacral spaces, in company with the 

 radiating ambulacral canals. 



The process of respiration is carried on partly by arborescent 

 gill-like organs placed round the mouth, which are of the 

 nature of greatly developed tube-feet, and which are not uni- 

 versally present ; partly by the tube-feet and their secondary 

 vesicles in general ; and partly by the vascular lining of the 

 test and the mesentery. The sea- water is admitted to the 

 body-cavity principally through the " madreporiform tubercle," 

 only a portion of the area of this being occupied by the stone- 

 canal; though, as previously remarked, recent observations 

 would go to show that this view is incorrect. 



The sexes are distinct in all the Echinoidea, and the repro- 

 ductive organs are in the form of five membranous sacs, which 

 occupy the inter-ambulacral areas, and open on the exterior by 

 means of the apertures in the genital plates. In the " irregular " 

 Echinoids (such as the " Heart-urchins ") there are only four 

 genital glands, and therefore only four genital plates in the 

 apical disc. 



