128 MANUAL OP ZOOLOGY. 



system. From the anal vessel proceed five arterial brandies, 

 which run along the ambulacra! spaces, and return their blood 

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

 direction. 



The nervous system consists of a ganglionated circular 

 cord, which surrounds the gullet below, or superficial to, the 

 4 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 

 Tiedemann, the water gains access to the interior by means of 

 ten 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. 



The sexes are distinct in all the Ecliinoidea^ 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. 



The Echinoidea may be divided into the following principal 

 families : 



Fam. I. Cidaridfs. 



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

 opposite the mouth, situated within the apical disc, surrounded by the 

 genital and ocular plates. Dental apparatus well developed. Am- 

 bulacra continuous from pole to pole (ambulacra perfecta). 

 Fam. II. Cly pea strides. 



Body discoid ; mouth central, or sub-central ; anus posterior, mar- 

 ginal, or infra-marginal. Genital plates with the madreporiform 

 tubercle in their centre. Dental apparatus well developed. Ambulacra 

 forming a five-rayed rosette on the dorsum of the test (ambulacra 

 circumscripta). 

 Fam. III. Spatangidce. 



Body usually cordiform ; mouth excentric. Anus posterior, and 

 supra-marginal. No dental apparatus. Ambulacra circumscript. 



