442 EADIATA : ECHINOPERMATA. 



V 



The circulatory apparatus is well developed, the blood 

 being distributed in real blood-vessels. 



Respiration is performed by means of branchiae, by 

 organs performing other functions, and by water passing 

 into the cavity of the body, and thus aerating the blood 

 through the capillary vessels of the viscera. 



The muscular system is well developed. The nervous 

 system consists of a ring around the commencement of 

 the oesophagus, which sends off branches along the rays. 



Echinoderms increase by means of eggs, are marine, 

 and are abundant on almost every coast ; and the remains 

 of extinct species fill the rocks in many regions. 



The Echinodermata are divided into five Orders : 



1. HOLOTIIUEIOIDEA or those with a long and more or less cylin- 



drical body covered with a tough skin containing calcareous 

 particles ; as Sea-Cucumbers, etc. 



2. ECHINOIDEA. or Echinoderms with a hard shell covered with 



spines ; as Sea-Urchins. 



o. ASTERIOIDEA or Echinoderms which are star-shaped, and 

 covered with a skin filled with movable plates, and bearing 

 tubercles instead of spines ; as Star-fishes. 



4. OPHIUEIOIDEA or star-shaped Echinoderms, whose long, 



slender, brittle arms start off abruptly from the relatively 

 small disk, and taper like a snake's tail ; as Serpent-stars. 



5. CEINOIDEA or Echinoderms which, either in their larval or 



adult form, are provided with a stem ; as Criuoids, both living 

 and fossil. 



SUB-SECTION II. 

 THE ORDEE OF HOLOTHURIOIDEA OR HOLOTHURIANS. 



THE Holothurioids are echinoderms which have the 

 body long, cylindrical, somewhat worm-like in general 

 appearance, with a row of appendages around the oral 

 opening, and without a calcareous shell, but with a tough, 



