annuloida: echinodekmata. 151 



the rays. The larva is venniform, and has no pseud embryonic 

 skeleton. 



The skeleton of the Asteroidea is. composed of a vast number 

 of small calcareous plates, or ossicula, united together by the 

 coriaceous perisome, so as to form a species of chain-armour. 

 Besides these, the integument is abundantly supplied with 

 spines, tubercles, and " pedicellarise." Lastly, the radiating 

 ambulacral vessels run underneath a species of intern^ skele- 

 ton, occupying the axis of each arm, and- composed of a great 

 number of bilateral " vertebral ossicles " or calcareous plates, 

 which are movably articulated to one another, and are provided 

 with special muscles by which they can be brought together or 

 drawn apart The upper surface of a star-fish corresponds to 



Fig. 42. — Crihella oculata. (After Forbes.) 



the combined ihter-ambulacral areas of an Echinus, and ex- 

 hibits the aperture of the anus (when present), and the " mad- 

 reporiform tubercle," which is situated near the angle between 

 two rays. The inferior or ventral surface corresponds to the 

 ambulacral areas of an Echinus, and exhibits the mouth and 

 ambulacral grooves. 



The mouth is central in position, and is not provided with 

 teeth ; it leads, by a short gullet, into a large stomach, from 

 which a pair of sacculated diverticula are prolonged into each 

 ray. A distinct intestine and anus may, or may not, be pre- 

 sent ; but the anus is sometimes wanting (in the genera, Astro- 

 pecten, Ctenodiscus, and Luidid). 



The aftibulacral system is essentially the same as in the 

 Echinoidea, and is connected with the exterior by means of the 



