TI1K ASTERIDKA 475 



canal lie superficial to these ossicles. There aie no oral ten- 

 tacula. 



The fivr-nivod body of the commonest of British Star- 

 fishes, 1 thi 1 Kivc-finger (Uraster, or Aster acanthion, rubens\, 

 presents an oral face, in the centre of which the mouth is 

 placed, and ,-in opposite or aboral face. The hardly-discern- 

 ible anal aperture is situated not exactly in the centre of this 

 face, but close to it. The mouth, which varies very much in 

 size, lies in the middle of a soft membranous oral disk. A 

 deep furrow, the ambulacral groove, occupies the middle of 

 the oral surface of each ray, and is nearly filled by contractile 

 sucker-like pedicels, with circular discoidal ends, apparently 

 arranged in four longitudinal series. The deepest part of the 

 groove is at its central end, where its lining passes into the 

 oral membrane. The shallowest part is at its distal end, where 

 it terminates against a median projection, the peduncle of 

 the eye, on the aboral side of which is the single median 

 ocular tentacle. Lines drawn from the mouth along each 

 ambulacrum are termed radii, and the regions occupied by 

 the ambulacra are said to be radial. The parts of the body 

 situated between the ambulacra are interradial. The lateral 

 \\.-ills of the ambulacral grooves of adjacent ambulacra unite 

 at the circumference of the oral disk, and give rise to five 

 interradial angles. On one side of the aboral face of the 

 centre of the body, between the origins of two of the rays, 

 and therefore interradial in position, is an oval or somewhat 

 pentagonal, slightly convex, porous plate, the surface of 

 which is covered with narrow, meandering grooves. This is 

 the madreporic tubercle, or madreporite. 



The perisoma, or wall of the body, upon the aboral face, 

 and upon the sides of the rays, is everywhere covered with 

 short spines. In the intervals between these, groups of deli- 

 cate membranous tubuli, which are closed at their free ends, 

 project. Small two-pronged, pincer-like bodies, the pedicel- 

 larice, are attached to the spines and to the perisoma between 

 them, and during life are seen to twist about and snap. 



The perisoma presents, externally, a cellular ectoderm, 

 provided with a thin cuticle, which bears numerous cilia. Be- 

 neath this lies a mesoderm, containing connective and mus- 

 cular elements, in which the calcareous structures which con- 

 stitute the skeleton are lodged. On the inner side of the 

 perisoma, a ciliated epithelium lines the perivisceral cavity. 



1 ( 'ompare Hoffmann, " Zur Anatomie der Asteriden." (" Niederlandisches 



Aivl.iv." Kd. ii., 1874.) 



