ECHINODEEMATA. 



103 



or skeleton, below whicli is placed the radiating ambulacral 

 vessel. All the internal organs are contained within the disk, 

 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 disk, and opens into a globular, 

 simple stomach, which is not furnished with an anal apertiu-e, 

 all indigestible 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 under stones 

 at low water on most shores. 



Fig. 35. — Comaiula rosacea, a Free adult ; & Fixed young (after Forbes). 



Oedee IV. Ceinoidea (Gr. krinos, a lily ; eidos, form). — 

 In this order are comprised JEchinodermata, 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 



