ECHINODERMATA. 101 



body, are hollow, and contain prolongations from the stomach. 

 The body is not enclosed in an immovable box or test, as in 

 the sea-urchins, but the integument is of a leathery nature, 

 and is richly furnished with calcareous plates, tubercles, and 

 spines. The true star-fishes are distinguished from the nearly 

 allied brittle-stars ( Ophiuroidea) by the fact that the arms are 

 direct prolongations of the body, that they contain prolonga- 

 tions of the stomach, 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 in- 

 ternal skeleton. The upper surface of the body and arms is 

 richly furnished with calcareous matter, in the form of prickles, 

 tubercles, spines, and pedicellarias, these last being peculiarly- 

 modified spines. The upper surface, also, exhibits the madre- 

 poriform tubercle in the form of a concentrically-striated disk 

 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 by a short gullet into a stomach which 

 usually terminates on the upper surface by an anal aperture ; 

 but this is occasionally wanting. From the stomach in all 

 the Asteroidea proceeds a series of much-branched membra- 

 nous sacs, two of which are prolonged into each ray. The 

 water-vascular or ambulacral system is in most essential re- 

 spects identical in structure with that of the sea-urchins, 

 making due allowance for the different shape of the body. 

 The nervous 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 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-fish ( Ur aster 

 rubens) the disk is small, and is furnished with long, finger-like 

 rays, which are properly five in number. In the Cribettce (Fig. 

 33) the general shape is much the same. In the sun-stars 

 (Solaster) the disk is large and well marked, the rays are from 

 twelve to fifteen in number, and they are shorter than the 

 diameter of the disk. In the cushion-stars (Goniaster) the 

 body is in the form of a five-angled disk, more or less flattened 

 on both sides, the rays being only marked out by the ambula- 

 cral grooves upon the lower surface. 



OKDEK HI. OPHIUROIDEA (Gr. ophis, snake; our a, tail; ei- 



