ECHINODERMATA. ASTEROIDEA. 67 



and coming between the dorsal and ventral surfaces of the 

 arms there are one or two rows known as the marginal 

 plates (c, d}. The dorsal surface of the skeleton is formed 

 of a meshwork of calcareous rods with membranous inter- 

 spaces. Short spines are often abundant on the plates. 

 Pedicellariae also occur, especially on the dorsal surface ;' 

 they are spines having a pincer-like termination, and 

 probably serve to remove excrement from the surface of 

 the body. Projecting into the mouth at the angles formed 

 by the ambulacral grooves are five pairs of plates, which 

 give the mouth a star-shaped form. The mouth leads 

 into the globular stomach, from which a small projection 

 passes a short way into each arm, the stomach narrows 

 and passes into the pentagonal pyloric sac, from the 

 angles of which five processes are given off, and pass into 

 the arms near the aboral surface and soon divide. From 

 the pyloric sac a short narrow intestine leads to the anus. 

 The water-vascular system consists of a vessel forming a 

 ring round the mouth (fig. 16 a), from which five branches 

 (6) radiate off, one down each arm, placed in the ambula- 

 cral groove and consequently outside the skeleton. Each 

 of these radial vessels gives off two rows of processes on 

 opposite sides, which pass on the one hand through the 

 pores between the ambulacral ossicles into vesicles situ- 

 ated dorsally to the ossicles and known as the ampullce (g} } 

 and on the other hand into the tubular processes known 

 as the tube-feet (f), which project along the ventral 

 surface of the arm and are provided at their extremities 

 with sucking discs. It is by means of these tube-feet 

 that the animal moves. The ampullae contract, forcing 

 water into the tube-feet, which then expand and become 

 attached to some foreign object, afterwards the ampullae 

 become distended causing the tube-feet to contract. The 



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