144 MANUAL OF ZOOLOGY. 



opposite the mouth, situated within the apical disc, surrounded by the 

 genital and ocular plates. Dental apparatus well developed. Am- 

 bulacra continuous from pole to pole (ambulacra perfecta}. 



Fam. II. Clypeastridce. 



Body discoid ; mouth central, or sub-central ; anus posterior, mar- 

 ginal, or infra-marginal. Genital plates with the madreporiform 

 tubercle in their centre. Dental apparatus well developed. Am- 

 bulacra forming a five-rayed rosette on the dorsum of the test (ambu- 

 lacra circumscripta'). 



Fam. III. Spatangidce. 



Body usually cordiform ; mouth excentric. Anus posterior, and sup- 

 ramargmal. No dental apparatus. Ambulacra circumscript. 



CHAPTER XX. 



ASTEROIDEA AND OPHIUROIDEA. 



ORDER ASTEROIDEA (Stellerida.} This order comprises the 

 ordinaiy star-fishes, and is defined by the following characters : 

 The body (fig. 40) is star-shaped or pentagonal, and consists 

 of a central body or " disc," surrounded by five or more lobes, 

 or " arms," which radiate from the body, are hollow, and con- 

 tain prolongations of the viscera. The body is not enclosed 

 in an immovable box, as in the Echinoidea, but the integument 

 ("perisome") 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 

 tube-feet are protruded from grooves on the under surface of 

 the rays. The larva is vermiform, and has no pseudembryonic 

 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 " pedicellariae." 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 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 

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

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



