42 ECHINODERMATA. 



with five or more branching arms, not connected with the visceral 

 cavity. 



This order abounded in Palaeozoic times. Comatula, the com- 

 monest of the recent forms, is free when mature. The mouth is 

 on the ventral surface ; the arms constantly in motion cause a 

 sufficient quantity of organic matter in solution to pass into it for 

 food. Some of the extinct forms had a proboscis or tube, often 

 of great length, arising between two of the arms. 



There are two families, the first permanently stalked, the other 

 free in the adult. J. Miiller, however, divides the Crinoidea into * 

 " Articulata" and " Tessellata," the latter comprising Platycrinus, 

 Actinocrinus, and some others, all extinct. 



Encrinida. Hyocrinus. ComatulidcB (Feather- 



Pentacrinus (Sea-lily). *Encrinus. stars). 



Holopus. *Apiocrinus. Comatula=Antedon. 



Ophiocrinus. *Platycrinus. Phanogenia. 



Ehizocrinus. *Actinocrinus. Actinomttra. 



Bathycrinus. *Saccosoma. 



Order III. BLASTOIDEA. 



Body rounded, enclosed in solid polygonal plates, and furnished 

 with a jointed stalk ; the summit of the body, or calyx, with five 

 areas longitudinally grooved. ]S T o pyramid. Palaeozoic. 



There are about fifty species ; the majority appear to have lived 

 in the Carboniferous period. 



*Pentatremites. *Ela2acrinus. *Eleutherocrinus. 



Class II. STELLERIDA. 



ASTEROIDEA. 



Body star-shaped or pentagonal, composed of a disk, either five 

 or more lobed, or with five or more arms. Mouth central, infe 

 rior, without teeth. Sexes distinct. 



The integument is coriaceous, strengthened by a vast number 

 of calcareous plates. One or more madreporiform tubercles or 

 plates are connected with the ambulacral water-system. The anal 

 aperture is either dorsal or absent. A blood-vascular system is 

 present in most. 



Fossil species are numerous, extending from the Lower Silurian 

 Asteracanthion, which still exists, up to the present time. Goni- 

 aster and Astropecten are found in the London Clay. 



