266 
ON THE ECHINODERMATA. 
Cystoidea have a more or less spherical body, supported on a jointed 
stem ; the basiform calyx is formed of close-fitting plates, of a poly¬ 
gonal figure, and varying in number in different genera, investing the 
surface like a coat of mail, except above, where there are three open¬ 
ings—one for the mouth, one for the vent, and one with a valve for the 
reproductive organs ; the fourth aperture is below, and is continuous 
with the canal in the stem. Some have two or four arms, others are 
armless. Certain species possess articulated tentacula and curious 
comb-like appendages, or pectinated rhombs in connection with the 
plates. This order is extinct, and their remains are found in the 
Devonian and Silurian rocks. Pseudu-crinites bifsasciatus is a good 
type of this order. 
Ophiuroidea. —The body is discoidal, distinct and depressed, pro¬ 
vided with long slender arms, in which there is no excavation for any 
prolongation of the viscera. They are, in fact, special organs for 
locomotion, independent of the visceral cavity. They have spines 
developed from their sides, which form highly movable and import¬ 
ant aids for locomotion. The mouth is always on the lower surface 
and is central, and surrounded by tentacula. The skeleton is 
extremely complicated, and composed of numerous calcareous pieces, 
which vary in number, size, shape, in the different genera. The long, 
slender, snake-like arms are supported by a number of vertebral-like 
pieces, which form the pliable rays for locomotion only. The common 
sandstar {Ophiura texturata), Lamk, is a typical form which well 
represents the order. 
Asteroidea have a depressed stelliform body, provided with five or 
more rays, or hollow arms, which are continuations of the body, and 
contain prolongations of the viscera. The mouth is always below, and 
central; some have a vent opening on the upper surface. Several 
rows of tubular retractile suckers occupy the centres of the rays. The 
skeleton is complicated and composed of numerous solid calcareous 
pieces joined together, and movable on each other, by which strength 
and flexibility is at once provided for; the outer surface is coriaceous, 
or studded with calcareous spines of various forms and sizes. A 
singular body, called the ^madreporiform plate, inasmuch as it 
resembles a miniature head of madrepore coral, is situated between 
two rays, and is in connection with a canal, through which water 
passes into the aquiferous system, the plate acting as a sieve to strain 
off all impurities from entering the channels. The suckers are 
tubular organs erected by injecting water into them, and by these they 
move slowly along. The nervous system consists of a cord of nervous 
matter which surrounds the mouth, and has ganglia or nervous centres 
opposite each ray, where branches are given off to the organs and a 
branch is sent out to the end of the rays where the eyes are placed. 
The stomach is very capacious, and sends a prolongation into each ray, 
so that each ray is, in fact, a portion of the body. The blood is cir¬ 
culated in one system of vessels and the water in another. The 
ovaries are very large, and produce an enormous number of eggs, the 
