HOW SEA-URCHINS GROW. 
95 
- 40- 
twisting and snapping incessantly, the effect is more 
comical than can be expressed in a description. 
But our sea-urchin is something more than amus- 
ing, he is a most wonderful example of how animals 
can be built upon the same plan, and yet so altered to 
suit their life that we should scarcely recognise them 
as relations. Looking at a sea-urchin, who would be- 
lieve that it has anything in common with the star- 
fish ? Yet if you examine it without its spines, a rough 
description will soon explain how alike they are. 
Suppose you were to take a dead star-fish and 
bend its rays backwards till they meet round the 
disc of the back ; sew the 
tips there, and then sew 
the five rays together up 
the sides so as to form a 
ball flattened in the mid- 
dle, you would then have 
the mouth of the animal 
(m, Figs 37 and 40) un- 
derneath the ball, and the 
five rows of feet (A, Fig. 
36, /A, Fig. 40) running A Sea . Urchin after its spines have 
Up it, While the edge of been rubbed off. 
each ray where there are m , Mouth, fh, Foot -holes 
no feet would touch throu g h which the alkin g tubes 
pass, wn, The water hole, e, Eyes, 
the edge of the next S} Sockets of larger spines. 
ray, making two rows of 
footless strips between each group of suckers. If 
you could now blow out this ball so that the mouth 
and back were some distance apart and the whole 
was round, this would roughly represent our sea- 
urchin without its spines. 
