472 



Marvels of the Universe 



upon any available part of an enemy their bite is a poisoned one. Experiments made to test the 

 virulence of the poison have shown that a single one made to bite a frog has caused the \-ictim's 

 heart to stop beating. The three-jawed organs, which are b}- far the most numerous, open quickly, 

 and close with a snap. One of their special functions is to seize the swimming forms of parasites 

 that are ever seeking to establish themselves on the Urchin's delicate exterior. A third kind end 

 in a pair of toothed jaws that give them a close resemblance to the head of a snake. Their function 

 appears to be to catch small animals as food, and hold them until the tube-feet can bend that way 

 and convey the captive from one to another until it reaches the Urchin's mouth. 



The Urchin's box is full of wonders, although the Urchin is an animal of a low grade in the scale 

 of life. It is a creature without a head or a brain, a heart or circulatory sj^stem. There is a nervous 

 system, but it is not of a high order. The complicated masticatory apparatus, which is known as 



" Aristotle's Lantern " (shown on 

 page 474), is worked by a ring of 

 muscles ; the five large teeth, 

 whose tips show externally in the 

 centre of the underside, form the 

 chief part of this apparatus. 



The chief food of the Common 

 Sea-Urchin appears to be the 

 leathery brown fronds of the great 



Tangles and the numerous small 



J'lioto fcw] 



[f. J. Spitlu. F.R..\f..'- 



URCHIN'S SPINE. 



animals that live upon them, among 

 other things small crustaceans and 

 worms. The fact that it is fre- 

 quentl\- found in crab-pots, attracted 

 thither by the pieces of fish pro- 

 vided as bait, shows that it is only 

 restricted to smaller animals natur- 

 ally as food by the fact that it 

 cannot capture larger ones. One 

 of our rat-er Urchins, with purple, 

 spines, when living upon exposed 

 limestone rocks, especially notice- 

 able on the west coast of Ireland, 

 excavates holes in the rock by the 

 rotation of its spines. These hollows 

 are sufficiently deep and wide to 

 accommodate the Urchin and save it from being washed away by heavy seas. Recent observa- 

 tions, however, seem to show that it does not start with solid rock, but takes advantage of an 

 existing crexdce and enlarges it to suit its own circumference. These internal matters, however, 

 ^ve must leave, and devote a little space to the forms of Urchins represented by our photographs. 



The Common Sea-Urchin, which may be regarded as the t3'pical form, bears about four thousand 

 spines, each a wonderful piece of workmanship. If one of these be microscopically examined it will 

 be seen to be a thing of great beauty, whether we examine it as a whole or in parts. Its full length 

 under a low power of magnification reveals itself as a beautiful tapering shaft, fluted in a manner 

 characteristic of the species ; the cup-like hoUow at its base, which revolves so freely on the polished 

 boss of the shell, is itself polished perfectly. If we cut the spine across and examine a thin slice 

 of it we shall see that in spite of its apparent solidity it is reallj- a symmetrical network of ribs 

 and hollows. We have said that there is not a hair's space between the edges of the living plates 



lified 



This is a highly 

 Each species has its own peculiar desisn. wh 

 in the groovins of the exterior. 



\v of a section cut across an Urchin's spine, 

 h corresponds with differences 



