560 THE ANIMAL KINGDOM. 



externally through it (?). There are two intestinal vessels, one the 

 ventral vessel coursing along the inner side of the tract, the other the 

 dorsal on the outer. They only extend a certain distance along the 

 intestine and are connected by capillaries. The ventral vessel opens into 

 the oral ring. The water-vascular ring is connected to radial trunks which 

 pass on the outer side of the masticatory organs, whilst the corresponding 

 blood-vessels are said to pass on the inner side upon the walls of the 

 pharynx in Desmosticha. The ring has five Polian vesicles in Desmosticha, 

 each placed in an interradius, and many vesicular appendages in Clypeas- 

 troidea. The Polian vesicles appear to contain blood -vascular plexuses. 

 The stone-canal is membranous in Echinus, calcareous in Cidaris, and 

 extends, as usual, to the madreporite. It has been stated recently that 

 it becomes continuous with the plexiform organ in Spatangus. The tube- 

 feet are connected to ampullae within the test, and are typically furnished 

 with a sucking disc supported by a calcareous rosette and by pieces in the 

 walls of the feet themselves. The feet belonging to the buccal plates of 

 the peristome end not in a disc but in two or more processes. The 

 dorsally placed feet in Desmosticha are often branched and respiratory. 

 The fine pores of Clypeastroidea emit minute feet of the ordinary structure, 

 the yoked pores of the petala flat pinnate respiratory feet. Similar pinnate 

 feet proceed from the pores of the petala in Petalosticha, a group in which 

 there are locomotive feet both with and without terminal discs. It is a 

 rule that the feet within an area inclosed by a fasciole differ from those 

 without it. Brown pigment corpuscles, apparently respiratory in nature 

 and containing iron, are met with in the coelome, water-vascular and blood- 

 vascular vessels, together with corpuscles of different colours. The buccal 

 or tegumentary gills, which are characteristic of the Ectobranchiate Des- 

 mosticha, are ten arborescent hollow diverticula of the coelome ; they pass 

 outwards in the peristome, each through a notch between the peristomial 

 ends of the ambulacra and interambulacra. In the Cidaridae or Ento- 

 branchiate Desmosticha five diverticula with lateral branches project radially 

 from the jaw-chambers between the alveoli into the coelome. Their 

 walls are supported as a rule by calcareous deposits, and their cavities 

 perhaps communicate with the exterior. They have also been found in a 

 Diadema \ 



The digestive tract in the Desmosticha and Clypeastroidea commences 

 with a pharynx surrounded by a complex masticatory organ, termed in the 

 former 'Aristotle's lantern.' This apparatus consists of an interradial por- 

 tion viz. five sharp-pointed teeth supported by five sockets or alveoli, 

 each composed of a right and left half covered above by corresponding 

 epiphyses, and of a radial portion viz. the rotula and the radii. The alveoli 

 are massive and placed horizontally in Clypeastroidea, vertically in Desmo- 



1 Stewart, Tr. L. S. (2), i. 1879. 



