138 



LIFE BY THE SEASHORE. 



though to seek protection. It is in consequence not very 

 easily seen except by careful search, but is common enough 

 in the Laminarian zone. The depressed shape and green 

 and purple colour make it easily recognised. As already 

 indicated, the common urchin only occurs somewhat sporadi- 

 cally between tide-marks, but it is at times thrown on shore 

 in great numbers after gales, and is generally to be found 

 in the dry condition on the beach. The diet of both 

 urchins seems to vary, probably in part according to the 

 locality; in many places both live largely on seaweed, but 

 are not averse to mingling this with animal matter. 



In both cases 

 the internal anat- 

 omy is very in- 

 teresting, and a 

 general notion of 

 its main outlines 

 is easily obtained. 

 With a strong 

 pair of scissors 

 make a circular 

 incision midway 

 between mouth 

 and anus, and then 

 lift off the upper 

 segment. In it 



FIG. 45. Echinus esculentus. common sea-urchin. The nr > p PPQ flip fiva 



r' ies have been removed from half the test, to u /& . MM 



w the structure of the latter. The reference reproductive or- 

 lines (a) inclose an ambulacral area; i is an inter- vowin^ 



ambulacral area. & clllb > Vcll j lli 5 



greatly in size ac- 

 cording to the season of the year; in the lower we see 

 Aristotle's lantern, which is very large relatively to the 

 size of the animal, and is perforated by the brown ali- 

 mentary canal, which, after leaving the lantern, coils about 

 the shell, and ultimately passes upwards to end at the 

 anus. Notice also the stone canal, a tube hanging verti- 

 cally from the madreporite, which opens into a ring canal 

 placed on the lantern, which again opens into five radial 

 canals running along the inner side of the ambulacral 

 areas. Each radial canal communicates by lateral branches 

 with the tube -feet, and with the leaf -like ampullw 



