BLASTOIDEA. 241 



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 usually ex-centric in position. The 

 last part of the gut is expanded to form an anal tube, which during life 

 is in constant movement, and has apparently a respiratory function. 

 From the cup, where the body cavity is in great part filled with con- 

 nective tissue and organs, four ccelomic 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 

 circumcesophageal 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 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, and a process suggestive of sexual union has 

 been observed in Antedon. 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. 



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 obviously 

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

 development in Silurian, Devonian, and Carboniferous times. 



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

 than that of other Echinoderms. 



The classification is a matter of considerable difficulty, but the old 

 division into Palseocrinoidea and Neocrinoidea must apparently be 

 abandoned. The recent forms include the stalked Pentacrinns, Rhizo- 

 crinus, etc., and the free Comatulids, which pass through a stalked 

 Pentacrinus stage, e.g. Antedon. 



Holopus is a remarkable deep sea form, with direct ancestors in the 

 Upper Silurian. Marsupites is an extinct Crinoid which had no stalk. 



Class Blastoidea. Wholly extinct. 



The Blastoids are first found in the Upper Silurian, later than Cystoids- 

 and Crinoids; they had their golden age in the Carboniferous and 

 Devonian times, but then disappeared. Their body was ovate, with 

 five ambulacral areas, with each groove of which jointed pinnules were 

 associated. 

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