106 INVERTEBItATE ANIMALS. 



■which may be simple or branched, but which do not contain any 

 prolongations fnmi the stomach, nor have their under surfaces 

 excavated into grooves for the protiusion of anibulacral tube-feet. 

 The arms, in fact, are not prolongations or lobes derived from the 

 body itself, liut are special appendages added for jiurposes of loco- 

 motion and prehension. The arms are very much longer than the 

 diameter of the disc, ami aie jjrotected by four rows of calcareous 

 plates — one above, one below, and one on each side. In the centre 

 of each arm is a I'ow of calcareous pieces which form a kind of 

 internal axis or skeleton, below wliich is placed the radiating 

 arnbulacral vessel. The main internal organs are contained within 

 the disc, and none of them jiass into the arms except the nerve- 

 cords and ambulacra! vessels. The mouth is placed in the centre of 

 the under surface of the disc, and opens into a globular, simple 

 stomach, which is not furnished with an anal aperture, all indigest- 

 ible particles being got rid of through the mouth. In various 

 points of their anatomy the Opiiiuroideii differ considerably from the 

 true Star- fishes, to which they are most nearly related, but these 

 <lifferences do not recjuire further notice. 



The habits of the Brittle-stars and Sand-.stai'3 are various, but 

 many of them iw.iy be found in rock-pools or under stones at low 

 water on most sliores. One of tlie most remarkable types is the 

 Medusa-head Star, in which the arms are divided fro)u the base, 

 first dichotomously and then into many l.>ranches. 



Order IV. Crinoidea. 



In this order are comj)rised Echlnoderiivita, in which the body ia 

 fixed, during the whfjle or a portion of the existence of the animal, 

 to submarine olijects by means of a jointed flexible stalk or column. 

 The Crinoidea were formerly very numerous, both individually and 

 in types, but they are rejiresentod at the pi-eseut day by but a small 

 nmuber of living forms, of which one group only (viz., that of the 

 Feather-stars) is at all of common occurrence. The body in the 

 Crinoids consists of a central disc or cuji formed of calcareous plates, 

 and protecting the b(jdy of the animal. From the margins of this 

 cup sjiring five or more arms which are ai'ranged in a radiating 

 manner, so as lo form a more or less feathery crown. In one of the 

 living grou|is (('niiKitiihi or Anti'doti), the animal, when full grown, 

 is free ; but in all other living tyjies, and in the great majority of 

 fossil forms, the body was attached through(jut life to the sea- 

 bottom by means of a jointed stalk fixed to the lower surface of the 

 cup (fig. f;7). 



The commonest living group is that of the Featlier-stars {Comatula 



