Clans 2. Asteroidea. Order 1. Asterida. 



133 



being thrown out again from the mouthy for the anus plays only a 

 subordinate part. 



The species of Starfish are numerous, and occur in all seas. The 

 following may be cited as examples : 



1. Asterias ruhens, a five-rayed form, witt tte tube-feet arranged in foui' 

 rows, and each, foot fumislied witli a sucker. Very common in North European 

 seas, occurring on the shores and to considerable depths. It is inimical to Oyster 

 beds, also causing havoc by devoui-ing Fish caught in nets or on hooks. Those 

 from deeper water attain a breadth of 50 c/m., the littoral ones are much 

 smaller. 



2. Solaster, Starfish of considerable size, with a large number of arms (about 

 ten), tube-feet with suckers, in two rows. In North European seas. 



Order 3. OpMurida {Brittle Stars). 



The arms, usually five in number, are long and narrow, and do 

 not meet at their bases ; the edge of the disc between each two arms 

 is usually straight, or somewhat 

 bulging. In addition, the dorsal 

 walls of the arms, by a different 

 arrangement of the ossicles, generally 

 difier somewhat in appearance from 

 that of the disc; so that they seem 

 to be well marked off from this, 

 especially when examined from above. 

 They differ, further, from those of the 

 Starfish, in the absence of ambulacral 

 grooves; the ventral surface is flat, 

 and usually covered with calcareous 

 ossicles which lie ventral to the radial 

 water- vessel ; dorsal to it are vertebra- 

 like ossicles, which constitute the 

 chief part of the arm, and are similar 

 to those of the Starfish, although 

 developed somewhat differently. The 

 tube-feet, which are without suckers, 

 are arranged in two series on the 

 ventral surface, near the margin of 

 the arms; and on the disc, near to 

 the mouth. The aboral surface of 



the arms is usually covered with large calcareous plates, that of the 

 disc is generally soft, with larger or smaller calcifications. The 

 arms are very flexible, and are capable of a serpentine movement. 

 The circular mouth lies in a stellate depression, the corners of 

 which are in the inter-radii, and are furnished with denticles. 

 The stomach is a wide sac, occupying the greater portion of the 



Fig. 88. Diagram explaining the 

 structure of an OpHurid, seen from 

 below, k slit-like aperture of a bursa, 

 m madreporite, o mouth, p one of the 

 ossicles from the ventral surface of the 

 arms, s tube-feet, s' point of origin of 

 one of them, s" tube-feet round the 

 mouth, ta denticles. — Orig. 



