230 OYSTERS, AND ALL ABOUT THEM. 



mies is the Echini, " sea-eggs," " sea-urchins," whose well 

 known empty cases are so common on every shore. The 

 Echini, in form, may be likened to melons, and, although 

 differing so much from the asterias, yet belong to the same 

 class the Echinoderms. The body of the sea-urchin 

 consists essentially of an exterior shell, or solid corona 

 covered with spines, and invested in a delicate membrane 

 furnished with vibratile cilia. This corona is formed of an 

 assemblage of contiguous polygonal plates, adhering to- 

 gether by their edges. The plates are so arranged that 

 the shell is divided into vertical zones hence its resem- 

 blance to the melon. These zones are of two kinds, one 

 being very much larger than the other ; the plates of the 

 larger zones are covered with sharp spines, which are 

 movable, and serve at once for protection and locomotion. 

 The plates of the smaller zones are pierced with pores, 

 from which issue filaments, by which the animal breathes 

 and walks. 



" It can travel either on its back or stomach. Whatever 

 their posture, they have always a certain number of feet 

 which carry them, and suckers with which they attach 

 themselves. In certain circumstances the animal walks 

 by turning upon itself, like a wheel in motion." (e) 



" Nothing is more curious than to see a sea-urchin walk 

 upon smooth sand. But for its colour, it might be mis- 

 taken for a chestnut with its bristling envelope, the spines 

 serving as feet to put the little round prickly mass in 

 motion. They have even been observed to make very 

 considerable progress under these circumstances. One of 

 the most singular organs of this interesting animal is its 

 mouth. It is most curious. Placed underneath the body, 



(e) " World of the Sea," p. 164. 



