SEA-URCHINS, STARFISH, AND BRITTLE-STARS. 131 



ventral surface we have the mouth, which has a complicated 

 structure. The main points are indicated in Fig. 43, which 

 is a diagram of the parts in the sand-star (Ophiurd). The 

 actual mouth -opening is small, for from its margin five 

 triangular projections jut inwards, the apex of the triangles 

 being towards the centre. These projections arise be- 

 tween the arms, and as they do not touch one another, 

 the mouth-cavity consists of a small central space con- 

 tinued into five slits, the slits corresponding to the rays. 

 Into these slit-like spaces the first tube-feet of each ray 

 project, and function as tentacles. The minute structure 

 of the triangular projections is of some importance in 

 identifying species. Each consists of a basal plate, or 

 mouth-shield, and two lateral plates in contact through- 

 out the whole or part of their length. In some cases these 

 lateral plates bear small spines at their edges, so that the 

 mouth-clefts are fringed by spines. Such spines are called 

 mouth-papillae. Again, spines may be present at the apex 

 of the triangle, such spines being called tooth -papillae. 

 Finally, beneath the tooth-papillae, and within the mouth- 

 cavity, there may be smaller spines called teeth. The 

 madreporite in brittle-stars is on the ventral surface, and 

 not the dorsal as in starfish, and replaces one of the mouth- 

 shields. 



The common brittle-star (Opliiothrix fragilis), which in 

 most places is very common, is to be found under stones 

 and among tangles between tide-marks. Fair-sized speci- 

 mens measure three to four inches from tip to tip, but the 

 arms are very brittle, and break off at very slight provo- 

 cation. The colours are bright and variable, the rays being 

 usually banded, and the disc of a contrasting colour. Keds, 

 browns, and yellows are common tints, but the rays are 

 frequently violet or grey. The arms are flattened, and the 

 most characteristic point is the presence of long notched 

 glassy spines borne at the sides of the arms in bunches of 

 seven. Some other more minute points of structure may 

 be given as follows. There are no mouth-papillae, so that 

 the sides of mouth-clefts are unnotched, but there are 

 numerous tooth-papillae and teeth. On the dorsal surface 

 of the disc the radial shields are very conspicuous, but the 

 rest of the disc shows much variation in the presence or 



