ECHINODERMATA. 107 
in snapping together like the beak of a bird. They occur in 
many other Echinodermata, and their use is obscure. 
Locomotion is effected in the sea-urchins by a curious system 
of contractile tubes which are known as the “ ambulacral tubes” 
or “ tube-feet,” and which are appendages of the water-vascular 
system. The following is essentially the arrangement of the 
whole aquiferous system (see fig. 43, 2). From the madre- 
poriform tubercle on the largest of the genital plates there 
proceeds a membranous canal (/) by which the outer water 
is conducted to a central tube (2), 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 
yy, 
Fig. 43.—Morphology of Echinoidea. 1. Young (larva) of an Echinus: a Mouth; 
4 Stomach; ¢ Intestine; s Skeleton. 2. Diagram of Echinus—the spines and 
tube-feet are represented over a small part of the test, the blood-vascular system 
is cross-shaded, and the nervous system is represented by the black line: @ Anus; 
6 Stomach ; c Mouth; dand_/ Vascular rings round the digestive canal; ¢ Heart; 
g Test; % Nervous ring round the gullet; 7 Sand-canal; z Circular canal round 
the gullet 3 #2 # One of the radiating ambulacral canals ; o Tube-feet; 2 Second- 
. ary vesicles at the bases of the tube-feet ; 4 Spines; » Madreporiform tubercle. 
prevent foreign particles gaining access to the interior. From 
the “circular canal” round the gullet proceed five “radiating 
canals” which take their course towards the summit of the 
shell, underneath the ambulacral areas (wz). In its course each 
radiating canal gives off numerous short lateral tubes (0)— 
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-discs. 
The tube-feet can be distended with water by means of,a series 
