260 PHYLUM ECHINODEKMA. 



The nervous system consists (a] of a circumpral ring with ambulacra! 

 nerves, and (/>) of axial nerves up the ossicles on the opposite side of 

 each arm and connected with a peculiar "chambered organ" in the 

 interior of the centro-dorsal plate. 



Apart from the superficial epithelium, there are no sensory structure.^. 

 The ciliated food canal descends from the mouth into the cup, and 

 curves up again to the anus, which is on a papilla. The last part <>f 

 the gut is expanded to form an anal tube, which during life is in con- 

 stant movement, and has apparently a respiratory function. From the 

 cup, where the body cavity is in great part filled with connective tissue 

 and organs, four coelomic canals extend into each of the arms. They 

 communicate at the apices of the arms and pinnules, and currents pass 

 up one and down the other. 



The blood vascular system consists of a circumoral ring, which is 

 connected with a radial vessel under each ambulacral nerve, and with a 

 circum-oesophageal plexus. 



The water vascular system consists as usual of a circumoral ring and 

 radial vessels, but in several respects it shows remarkable modification. 

 The madreporite of other forms is represented by fine pores which open 

 from the surface of the calyx directly into the body cavity, and which 

 may be very numerous; there are said to be 1500 in Antedon rosacea. 

 By these pores water enters the body cavity, and from it enters the 

 numerous stone canals which hang from the ring freely in the body 

 cavity, and open into it near the pore canals. There are no Polian 

 vesicles or ampulloe, the tube-feet are small, are arranged in groups of 

 three, and are connected by delicate canals with the radial vessels. 

 Certain of them form tentacles around the mouth, and these are supplied 

 by canals coming off directly from the ring canal. 



The sexes are separate. The reproductive organs extend as tubular 

 strands from the disc along the arms, but are rarely functional except 

 in the //;/;/ ulcs, from each of which the elements burst out by one duct 

 in females, by one or two fine canals in males. 



The oval ciliated larva of Anfcdon, the only one known, is less 

 peculiar than that of other Echinoderms. 



There are about 400 living species in twelve genera, but about 1500 

 species in 200 genera are known from the rocks. The class is obviously 

 decadent. It is represented in the Cambrian, and attained its maximum 

 development in Silurian, Devonian, and Carboniferous times. 



The recent forms include the stalked Pentacrinus^ Rhhocriiius, etc., 

 and the free Comatulids, which pass through a stalked Pentacrinus 

 stage, e.g. Antedon. 



Class EDRIOASTEROIDEA. Wholly extinct. 



These extinct Pelmatozoa had a sac-like theca of an indefinite number 

 of irregular plates, with a mouth in the centre of the upper surface, 

 with at most a short stalk. Ordovician, Silurian, and Devonian. 

 " They are alone among Pelmatozoa in presenting a type of ambulacrum 

 from which the holothurian, stellerid, and echinoid types may readily 

 be derived " (F, A. Bather). 



