202 MANUAL OF ZOOLOGY. 



" arms" which radiate, from the body, are hollow, and contain 

 prolongations of the viscera. The body is not enclosed in an 

 immovable box, as in the Echinoidea, but the integument ("peri- 

 some"} is coriaceous, and is strengthened by irregular calcareous 

 plates, or studded by calcareous spines. No dental apparatus is 

 present. The mouth is inferior, and central in position ; the anus 

 either absent or dorsal. The ambulacral ttibe-feet are protruded 

 .from grooves on the under surface of the rays. The larva is 

 vermiform, 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 " pedicellarias." Lastly, the radiating 

 ambulacral vessels run underneath a species of internal skele- 

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

 number of bilateral "vertebral ossicles" or calcareous plates, 

 which are rnovably 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 the combined inter-ambulacral areas of an Echinus, and 

 exhibits the aperture of the anus (when present), and the 

 " madreporiform 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 pres- 

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

 pecten, Ctenodiscus, and Ltddia). 



The ambulacral system is essentially the same as in the 

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

 " madreporiform tubercle," or " nucleus," two, three, or more 

 of these being occasionally present. The conical or cylindrical 

 ambulacral tube-feet are arranged in two or four rows, along 

 grooves in the under surface of the arms (fig. 98). Each am- 

 bulacral groove is continued along the lower surface of one 

 of the arms, tapering gradually towards the extremity of the 

 latter. The floor of each groove is constituted by a double 

 row of minute calcareous pieces the "ambulacra! ossicles" 

 which are movably articulated to one another at their inner 

 ends. At the bottom of each groove is lodged one of the 

 radiating canals of the water-vascular system or ambulacral 



