GENERAL STRUCTURE OF STAB-FISHES. 109 



Order 1. Bntcltiata (True Crinoids). Calyx with large pinmikted 

 arms, without dorsal cnlical pores, mostly stalked (Encri- 

 nus, Pentacrinus, Apiocrinus, Rhizocrinus, Holopus, Ante- 

 dou, Actinometra, Phauogenia). 



Order 2. Blastoidea. Armless, but with five series of pinnulse, and 

 with a stalk (Pentremites. No living representatives). 



Order 3. Cystidea. Usually armed, with jointed pinnuloe, and a short 

 stalk, the latter sometimes absent, as in Caryocystites. (All 

 fossil forms, as Edriaster, Caryocystites, Sphferonites, etc.) 



Laboratory Work. The living Crinoids are great rarities, and few 

 students have access even to alcoholic specimens. The recent re- 

 searches on their internal anatomy have been made in large part by 

 cutting thin sections for the microscope, and staining them with car- 

 mine, etc., after the methods of the histologist. 



CLASS II. ASTEKOIDEA (Star-fishes). 



General Characters of Star-fishes. -- Having already 

 studied the structure of the common star-fish, we are pre- 

 pared to understand the classification of the class. The 

 star-fishes have star-shaped, flattened bodies, with round or 

 flattened arms, a madreporic plate, and two or four rows of 

 ambulacral feet. 



Order 1. Ophiuridea (Sand-Stars). This division is 

 characterized by the body forming a flattened disk, with 

 cylindrical arms, the stomach not extending into the arms, 

 and there is no intestine or anal opening. The ambulacral 

 furrow is covered by the ventral shields of the tegument, so 

 that the ambulacral feet project from the sides of the arm. 

 They have no interambulacral spaces or plates. The am- 

 bulacral feet or tentacles do not have a sucker at the end, 

 but are provided with minute tubercles. They move faster 

 than the true star-fishes, the arms being more slender and 

 flexible. The madreporic body is one of the large circular 

 plates in the interambulacral spaces around the mouth. 

 The external openings for the exit of the eggs form distinct 

 fissures or slits, one on each side of each arm. The ovaries 

 are situated in the body, not extending into the arms, the 



