104 INVERTEBEATE ANIMALS. 



stomacli, and that they are deeply grooved on their under surfaces 

 for the radiating vessels of the water-vascular system, which are 

 further protected by a sort of internal skeleton. The upper surface 

 of the body and arms is richly furnished with calcareous matter, in 

 the form of prickles, tubercles, spines, and pedicellarise, these last 

 being peculiarly modified spines. The upper surface, also, exhibits 

 the madreporiform tubercle in the form of a concentrically striated 

 disc placed at the angle between two of the rays ; and also the 

 aperture of the anus, when this is present. The mouth is placed 

 in the centre of the lower surface, and is not furnished with 

 teeth. It leads into a pouched stomach and very short intestine, 

 which usually terminates on the upper surface by an anal aper- 

 ture ; but the anus is occasionally wanting. From the upper 

 end of the stomach in all the Asteroidea proceeds a series of 

 much - branched membranous sacs, two of which are prolonged 

 into each ray. These sacs are of a brown or greenish colour, and 

 are often called the " hepatic cfeca," as they are supposed to corre- 

 spond with the liver. The water- vascular or ambulacral system is 

 in most essential respects identical in structure with that of the Sea- 

 lu'chins, making due allowance for the different shape of the body. 

 The madreporic plate leads into a short sand-canal, which opens 

 into a circular ring round the gullet. The circular ring carries 

 Polian vesicles, .uid gives off the radiating ambulacral vessels, one 

 of which is prolonged to the extremity of each ray, running at the 

 bottom of the deep ambulacral groove on the under surface of the 

 arms. The ambulacral groove is formed by a double row of cal- 

 careous pliites (the " ambulacral- ossicles "), and the radiating ambu- 

 lacral vessel with its tube-feet lies underneath these plates. On the 

 other hand, the "ampullie," or little bladders which distend the 

 tube-feet, are situated nhnte the ambulacral ossicles, and therefore 

 in the interior of the arm ; and the ampulte become connected with 

 their i-espective tube-feet by jiassing through a series of pores 

 formed between successive pairs of ambulacral ossicles. The ner- 

 vous system consists of a gangliated ring surrounding the mouth and 

 sending branches along each of the arms. The reproductive organs, 

 like the nervous system, exhibit a radiate condition, being arranged 

 in pairs in each ray. 



The Star-fishes are found on almost all shores, but many forms 

 are properly inhabitants of deep water. They differ much in the 

 general shape of the body. In the common Cross-fishes ( Uraster or 

 Asteriax) the disc is small, and is furnished with long finger-like 

 rays, which are properly five in number. In the Cribelloe the 

 general shajie is nuich the same. In the Sun-stars (Solaster) the 

 disc is large and well marked, and the rays are from twelve to fif- 



