GENERAL STRUCTURE OF STAR-FISHES. 191 
Order 1. Brachiata (True Crinoids).—Calyx with large pinnulated 
arms, without dorsal calical pores, mostly stalked (Encri- 
nus, Pentacrinus, Apiocrinus, Rhizocrinus, Holopus, Ante- 
don, Actinometra, Phanogenia). 
Order 2. Blastoidea.—Armless, but with five series of pinnule, and 
with a stalk (Pentremites. No living representatives). 
Order 3, Cystidea.—Usually armed, with jointed pinnule, and a short 
stalk, the latter sometimes absent, as in Caryocystites. (All 
fossil forms, as Edriaster, Caryocystites, Spheeronites, etc.) 
Laboratory Work.—Vhe 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. 
Crass JI.—AstTEROIDEA (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 
