CLASS ECHINODERMATA. 1 9 



the opening of the month, surrounded by a fringe of long 

 spines. From the mouth radiate five broad and shallow 

 grooves, which proceed along the under surface of the 

 five arms, gradually tapering towards their extremities. 

 Each of these grooves is bordered by a row of long spines, 

 and each has its floor formed by a double row of little 

 plates (fig. 7, a a), which run across the groove, and are 

 so shaped as to leave a little opening between every pair 

 of plates in the series. 



It follows from the above, that if we cut the arm of the 

 Star-fish across, we find that it is rounded above, and 



Fig. 7.— One of the arms of the Star-fish, cut across, to show the groove on 

 the under surface, a a Transverse plates which form the floor of the 

 groove ; b The water- vessel, with the little " feet " proceeding from it ; n 

 Nerve-cord. 



hollowed out or grooved below (fig. 7). If we examine 

 the animal in its living state, we find each groove 

 under the arms to be occupied with four rows of little 

 delicate membranous tubes, which end in little suckers. 

 These tubes are called the "feet," because it is by the 

 combined action of these that the Star-fish creeps about. 

 The "feet" can be thrust out to a great length, and they 

 spring from a common tube which runs along the bottom 

 of the groove below the arm. (They are shown in fig, 

 7 at h, but here only two of the rows of feet are shown, 

 the other two being omitted for the sake of clearness). 

 The tubes which run along the grooves under the arms, 



