90 INVERTEBRATE ANIMALS. 



CLASS I. ECHINODEEMATA. 



The members of this class are popularly known as sea- 

 urchins, star-fishes, brittle-stars, feather-stars, sea-cucumbers, 

 etc., and derive their name of Echinodermata (Gr. echinos, a 

 hedgehog ; and derma, skin) from the generally prickly nature 

 of their integuments. In all, the skin is possessed of the power 

 of secreting carbonate of lime, but in very different degrees. 

 In the sea-urchins this goes so far that the body becomes en- 

 closed in an immovable box, composed of numerous calcareous 

 plates firmly jointed together. In the star-fishes and their 

 allies the skin is rendered prickly by grains, tubercles, or 

 spines of calcareous matter, and the body is either destitute 

 of regular plates or is only partially enclosed by them. In the 

 sea-cucumbers, again, the calcareous matter is mostly only 

 present in the form of minute grains scattered in the skin. 

 When adult, they all show a more or less distinctly radiate 

 structure, which is most conspicuous in the star-shaped star- 

 fishes and sand-stars, but can be detected in all the members 

 of the class. When young, however, they almost always ex- 

 hibit what is called " bilateral symmetry " that is to say, they 

 show similar parts on the two sides of the body. In all Echino- 

 derms there is a water-vascular system of tubes, which is 

 termed the "ambulacral system," which generally communi- 

 cates with the exterior, and which in most cases is used in 

 locomotion. An alimentary canal is always present, and is 

 always completely shut off from the general cavity of the 

 body. A vascular or circulatory system is sometimes present. 

 There are always distinct organs of reproduction, which are 

 almost always placed in different individuals, so that the sexes 

 are distinct. The nervous system is in the form of a ring sur- 

 rounding the gullet and sending branches in a radiating man- 

 ner to different parts of the body. 



The Echinodermata are divided into seven orders, as fol- 

 lows: 



1. Echinoidea (Sea-urchins). 



2. Asteroidea (Star-fishes). 



3. Ophiuroidea (Sand-stars and Brittle-stars). 



4. Crinoidea (Feather-stars). 



5. Cystoidea (extinct). 



6. JBlastoidea (extinct). 



7. Holothuroidea (Sea-cucumbers). 



This is by no means a true arrangement of these orders, but 

 it is convenient to consider them in this sequence. 



