ANNULOIDA: ECHINODERMATA. 143 



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 pseudohsemal system of the Annelida, 

 rather than with the true circulatory system of higher animals. 

 By Huxley, however, the pseudohaemal vessels of the Annelides 

 are looked upon as homologous with the water-vascular system 

 of the Scolecida, to which the water-vascular or ambulacral 

 system of the Echinoderms is unquestionably comparable. 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, 

 branched processes, which are attached to the extremities of 

 the inter-ambulacral areas round the mouth ; but others deny 

 that these are perforated by any apertures. These processes 

 are apparently nothing more than greatly developed tube-feet, 

 and they are probably homologous with the crown of feathery 

 tentacles surrounding the mouth in the Holothurians ; though 

 this is perhaps represented by some very large tube-feet placed 

 just round the mouth. 



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. 



The Echinoidea may be divided into the following principal 

 families : 



Fam. I. Cidarida. 



Body nearly globular ; mouth on trie lower surface, central ; anus 



