274 PHYLUM ECHTNODERMA 



"centro-dorsal" ossicle, bearing the cirri; this conceals the coalesced 

 " basals" of the larva ; above this are three tiers of " radials," whence 

 spring the " brachials " of the arms. 



The oral disc, turned upwards, is supported by plates. Here the 

 anus also is situated. The arms usually branch in dichotomous fashion, 

 and thus ten, twenty, or more may arise from the original five. But the 

 growing point continues to fork dichotomously, like the leaf of many 

 ferns, and as each alternate fork remains short, a double series of lateral 

 * ' pinnules " results. The arms are supported by calcareous plates. The 

 stalk usually consists of numerous joints, especially in extinct forms, in 

 some of which it measured over fifty feet in length. Except in Holopus, 

 Hyocrinus, and in the stalked stage of Antedon^ the stalk bears lateral 

 cirri. 



The nervous system consists (a) of a circumoral ring with ambulacral 

 nerves, and (6) of axial ccelomic 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 structures. 

 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 of 

 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-cesophageal 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 Poll an 

 vesicles or ampullae, 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 pinnules , 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 Antedon^ the only one known, is less 

 peculiar than that of other Echinoderms. 



There are about 400 living species in twelve genera, but about 1 500 

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

 presented in the Cambrian, and attained its maximum development in 

 Silurian, Devonian, and Carboniferous times. 



