2 ECHINODERMA. 



main trunks of which always lie along the line of the rays ; these 

 radially-disposed tubes are in connection with one another by 

 means of a circular vessel which lies round the mouth, and is 

 connected with the outer world by means of the tube which is 

 known as the madreporic canal. These tubes contain a clear fluid, 

 and form the " water-vascular system " ; the main trunks are 

 ordinarily, though not always, provided with double rows of 

 " suckers " or " pedicels," which sometimes have circular terminal 

 disks, and sometimes end in a point ; so that their function is not 

 always ambulatory, but may be, at least, respiratory, Very various 

 names of varying degrees of cumbrousness or inappropriateness 

 have been given to these projecting structures, and I think it is 

 wisest to return to the old and simple expression " podia." 



s.c. 



Diagram of Water-vessels. 



c.c, circular canal, with p.v., its Polian vesicles; from it a radial canal (v.c.) is 

 given off along the lower surface of each arm ; this supplies, by side 

 branches, the suckers, s. ; connected with each sucker is a contractile 

 swelling or ampulla (a.). The circular canal is in connection with the 

 exterior by s.c, the madreporic or stone-canal, and opens to it by the 

 madreporite («?.). 



Similarly, the system of delicate vessels which convey a corpus- 

 culated and nutrient fluid (blood) through the body exhibits the 

 influence of the radial symmetry, and the same is true of the nervous 

 system ; like the water-vascular system, the trunks of each of these 

 systems in the several rays are combined into a single whole by 

 means of, respectively, a circular vessel and a circular trunk 

 disposed around the mouth. 



In most groups of Echinoderms the radial disposition of parts is 



