ECHINODERMATA. 99 



there proceeds a membranous canal by which the outer water 

 is conducted to a central tube, which forms a ring round 

 the gullet. The tubercle is spongy, and is perforated with 

 little holes, and its function is probably to act as a filter, and 

 prevent foreign particles gaining access to the interior. From 

 the " circular canal " round the gullet proceed five " radiating 

 canals " which take their course toward the summit of the 

 shell, underneath the ambulacral areas. In its course each 

 radiating canal gives off numerous short lateral tubes the 

 ambulacral tubes or tube-feet which gain the exterior of 

 the shell by passing through the apertures in the ambulacral 

 plates of the shell, and which terminate in little sucking- 

 disks. The tube-feet can be distended with water by means 

 of a series of little muscular bladders placed at their bases, 

 and they can thus be thrust far out beyond the shell, into 

 which they can be again withdrawn at the will of the animal. 

 However long the spines may be, the animal can protrude 

 the tube-feet to a still greater length ; and by the combined 

 action of the little suckers at their extremities locomotion is 

 effected with moderate rapidity, considering the bulk of the 

 body. 



The digestive system in the Echinus consists of a mouth 

 armed with a curious apparatus of calcareous teeth, which 

 opens into a gullet, which in turn conducts to a distinct 

 stomach. From the stomach there proceeds a long and con- 

 voluted intestine, which is attached to the interior of the 

 shell by a delicate membrane or " mesentery," and terminates 

 in a distinct vent. The surface of the mesentery, as well as 

 that of the lining membrane of the shell, is richly ciliated, and 

 thus serves to distribute the fluids of the body-cavity to all 

 parts of the body. In this way, also, respiration is subserved, 

 though it is probable that the chief agent in this function is 

 to be found in certain specialized portions of the ambulacral 

 system. The circulatory system consists in its central portion 

 of two rings placed round the opposite ends of the alimentary 

 canal, and united by an intermediate muscular cavity or heart. 

 The nervous system consists of a gangliated cord placed round 

 the gullet, and sending five radiating branches along the 

 ambulacral areas. The sexes are distinct, but in both the re- 

 productive organs are in the form of five membranous sacs 

 placed in a radiating manner in the interambulacral areas, and 

 opening at the genital plates. The embryo of the Echinus is 

 at first a little free-swimming ciliated organism, and it passes 

 through an extraordinary development, which can only be 



