1 62 MANUAL OF ZOOLOGY. 



round the interior of the shell, and terminates in a distinct 

 anus. The mouth is always situated at the base of the test, 

 and may be central, sub-central, or altogether excentric in 

 position. The anus varies considerably in its position, being 

 usually situated within the apical disc, and surrounded by 

 the genital and ocular plates, when the test is said to be 

 " 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 sub-marginal. The convolutions of the alimen- 

 tary 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. 57, 2) 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 blood-vascular system has been thought to be 

 homologous with the pseudohaemal system of the Annelida, 

 rather than with the true circulatory system of higher animals. 

 By Huxley, however, the pseudohsemal vessels of the Anne- 

 lides are looked upon as homologous with the water- vascular 

 system of the Scoledda, to which the water-vascular or am- 

 bulacral system of the Echinoderms is unquestionably com- 

 parable. The haemal system, therefore, of the Echinoderms 

 must be regarded as something not represented amongst the 

 Annelides. 



The nervous system consists of a ganglionated circular cord, 

 which surrounds the gullet below, or superficial to, the " circular 

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

 along the ambulacral spaces, in company with the radiating 

 ambulacral canals. 



There is no distinct respiratory organ, but the function of 

 aeration of the blood appears to be performed partly by the 

 vascular lining of the test and the mesentery, and partly by the 

 secondary ambulacral vesicles. The perivisceral cavity is filled 

 with sea-water, but the mode in which this is admitted, or 

 renovated, is not known with certainty. According to Tiede- 

 mann, the water gains access to the interior by means of short, 



