42 ECHINODERMS OF THE BRITISH ISLES 



wliole animal being pentagonal {e.g. Ceramaster^ Porania) — or they 

 may be very long and rather narrow, and then the body is quite 

 small {e.g. Asterias, Luidia). The relation between the length of 

 arms and the diameter of disk is of systematic importance ; the 

 distance from the centre of disk to the point of the arm is desig- 

 nated R, the distance from the centre to the edge of disk midway 

 between the arms, r. When it is stated, for example, that R = 5r, 

 it means that the distance from the centre of the disk to the point 

 of the arms is five times as large as the distance from the centre 

 to the edge of the disk. Mostly the arms pass gradually, without 

 marked limit, into the disk, but sometimes there is a distinct 

 constriction at the base of the arms {Asterias) ; in Brisinga the 

 limit between disk and arms is sharp and distinct. The 

 number of the arms is commonly 5, but in some forms {Solaster, 

 Brisinga) there are more (9-15) arms, and in some exotic forms 

 even many more (25-40), the number increasing with age. Of 

 normally 5-rayed species, specimens with 4 or 6 arms may occur. 

 Sometimes an arm may be bi- or tripartite. The arms have 

 an internal skeleton consisting of paired plates, the ambulacral 

 plates, which form a structure like a vertebral column. In some 

 forms {Astropecten) a small plate, the superamhulacral plate, is 

 found internally within the arms, one above each ambulacral 

 plate. On the under side of the arm there is an open furrow, 

 the foot-groove or ambulacral furrow, limited on each side by 

 a series of distinct plates, the adambulacral 2>lates, one to each 

 ambulacral plate. In the bottom of the furrow the tube- feet 

 are found, arranged in two series ; in some forms they are 

 arranged in four series, which is due to an alternating displace- 

 ment of the two original series. The tube-feet issue from 

 between the ambulacral plates. They are very extensile, soft, 

 and flexible, without skeletal parts, and generally terminate 

 in a sucking disk (wanting in Astropectinids) . The ampullce of 

 the tube-feet form very conspicuous series of vesicles along 

 the inside of the ambulacral plates, and are very easily seen on 

 opening the arms from the dorsal side ; they may be single or 

 double, a character of some systematic value. At the point of 

 the arm there is an unpaired tentacle, carrying on its underside 

 a red pigment-spot, a rather well-developed eye, provided with a 

 sort of lens. The point of the arms is always bent upwards so 

 that the eye is not obscured. At the point of the arm there is, 

 above the tentacle, a larger unpaired plate, the terminal plate. 



Along the edges of the ambulacral furrow, on the adambulacral 

 plates, are found some regularly arranged adambulacral or furrow 



