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 les 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 itis 
wisest to return to the old and simple expression ‘ podia.” 
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 inadreporie or stone-canal, and opens to it by the 
madreporite (7.). 
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 Kchinoderms the radial disposition of parts is 
