110 ECHINODERMA. ASTEROIDEA 



ampullce (g), and also into the tubular processes known as 

 the tube-feet (f), which project along the under surface of 

 the arm and are provided at their extremities with sucking- 

 discs. The discs become attached to foreign bodies, and 

 by means of the contraction of the tube-feet the animal 

 moves. The water- vascular system is placed in com- 

 munication with the exterior by a canal (fig. 43, d) passing 

 from the circular vessel to the aboral surface of the disc 

 and ending in the madreporic plate. This is known as the 

 stone-canal on account of the deposit of carbonate of lime 

 in its walls. On the aboral surface, and sometimes upon 

 other parts of the body also, there are tube-like projections 

 of the skin, which form simple respiratory organs known 

 as dermal branchice. 



The distribution of the main part of the nervous 

 system is similar to that of the water-vascular system. 

 It consists of a ring round the mouth and of a branch 

 which extends down the ambulacral groove of each arm ; 

 there is also a layer of fine nerve-fibres under the ectoderm. 

 At the tip of each arm is an eye-spot. The genital glands 

 occur in pairs at the base of each arm and open to the 

 exterior between the rays ; generally the openings are on 

 the aboral surface, but in a few cases on the oral surface. 



Palaeaster. Body pentagonal. Disc small. Arms thick, 

 convex, of moderate length ; upper surface formed of rows of small 

 ossicles provided with spines. Ambulacral ossicles alternating on 

 either side of the deep ambulacral groove. There is a row of 

 ad-ambulacral plates and a row of large marginal plates. Madre- 

 poric plate small. Bala Beds to Carboniferous. Ex. P. euckaris, 

 Devonian. 



Palaeasterina. Disc large, pentagonal. Arms short, but 

 distinctly marked off from the disc ; ambulacral ossicles alternating ; 

 marginal plates smaller than ad-ambulacrals and usually with small 



