106 INVERTEBEATE AXIMALS. 



which may be simple or branclied, but which do not contain any 

 prolongations from the stomach, nor have their under surfaces 

 excavated into grooves for the protrusion of ambulacral tube-feet. 

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

 body itself, but are special ajjpendages added for purposes of loco- 

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

 diameter of the disc, and are protected by four rows of calcareous 

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

 of each arm is a row of calcareous pieces which form a kind of 

 internal axis or skeleton, below which is placed the radiating 

 ambulacral vessel. The main internal organs are contained within 

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

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

 the under surface of the disc, and opens mto 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 Ophiuroidea differ considerably from the 

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

 differences do not require further notice. 



The habits of the Brittle-stars and Sand-stars are various, but 

 many of them may be found in rock-pools or xinder stones at low 

 water on most shores. One of the most remarkable types is the 

 Medusa-head Star, in which the arms are divided from the base, 

 first dichotomously and then into many branches. 



Order IV. C'rinoidea. 



In this order are comprised Echinodernutta, in which the body is 

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

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

 The Crinoidea were formerly very numerous, both individually and 

 in types, but they are represented at the present day by but a small 

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

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

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

 and protecting the body of the animal. From the margins of this 

 cup spring five or more arms which are arranged in a radiating 

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

 living groups {Comatula or Antedon), the animal, when full grown, 

 is free ; but in all other living tyj^es, and in the great majority of 

 fossil forms, the body was attached throughout life to the sea- 

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

 cup (fig. 67). 



The commonest living group is that of the Feather-stars (Comatula 



