ASTEROIDEA. 



167 



The form of body varies from that of a pentagonal disc in 

 which the rays are only marked by the angles (Pent agon - 

 asteridae, Fig. 117, Pterasteridae, species of Culcita), to that 

 of a star (Fig. 139), in which the disc is small and the 

 arms as sharply marked off from it as in the Ophiurids 

 (Brisingidae). 



The number of rays varies in living forms from five to forty-five (Labi- 

 diaster). Five is the most usual number and is especially constant in the 

 discoidal forms and in those with well-developed marginal plates 

 (Palmipes rosaceus is exceptional in having eleven and Culcita tetragona 

 in having four). Four is sometimes found as an individual variation. 

 The number of rays shows a distinct tendency to increase in families, in 

 which the arms are long, the disc small, 

 and the marginal plates feebly developed 

 (e.g. Brisingidae, Heliasteridae, Asterii- 

 dae, Echinasteridae). When there are 

 more than six rays individual variations 

 in their number are fairly common. In 

 Labidiaster in which the arms are very 

 numerous, the number of them increases 

 with the growth of the animal ; but in 

 most if not all other cases the full num- 

 ber is laid down in the embryo. 



The body is usually compressed 

 dorso-ventrally. On the actinal 

 surface, reaching the whole length 

 of the radii and terminating in the 

 centre of the disc in the oral area 

 or depression, are the ambulacral 

 grooves, and from them project the 

 two, more rarely four, rows of 

 tube-feet. The mouth is placed 

 on the actinal surface in the centre 

 of the disc in the oral depression. There are no circumoral 

 tentacles or tentacular prolongations of any kind round the 

 mouth, but at the distal end of each ambulacral groove there 

 is a red pigment spot which is called the eye or ocellus and 

 over which projects an unpaired tentacle-like structure ; this is 

 the ocular tentacle and. contains the end of the radial water- 

 vascular trunk. The dermal skeleton is well developed, and 

 carries, especially on the abactinal surface, numerous spines 

 and usually pedicellariae. The anus, which is absent in the 

 Astropectinidae and probably in the Porcellanasteridae, is on 



FIG. 117. Pentagonaster Parkinson. 

 Forbes (after Perrier) . Seen from be- 

 low and from the side. 



