SEA-URCHINS. 



169 



Sir W. Thomson gives a graphic account of the surprise occasioned by the move- 

 ments that passed through the test of Asthenosoma hystrix as it assumed upon the 

 deck what seemed its usual form and attitude. The test moved and slirank from 

 the touch when handled, and felt like a star-fish. It is quite dangerous to handle the 

 species of this family when alive, as the wounds they make with their numerous sharp 

 stinging spines produce a pain and numbness as unpleasant as that occasioned by the 

 stinging of a Portuguese man-of-war. The tests of some species (as P. tenue) attain a 

 diameter of six inches. This species has been taken in two thousand seven hundred 

 and fifty fathoms. Judging from the large size of the eggs and of the genital openings, 

 this group of sea-urchins is probably viviparous. 



The largest family of regular sea-urchins is that of the Echinidje, which may be 

 divided into the two groups of Echinometridae, in which the pores of the ambulacra! 

 areas are arranged in arcs of several pairs of pores, and Echinidas proper, which have 

 arcs consisting of only three pairs of pores. This difference is greater than it seems, 

 for the mode of growth of the bands of pores is quite unlike in the two groups. 



Colobocentrotus atratus is covered by a pavement of closely packed hexagonal 

 spines, completely concealing the surface of the test. Those at the edge of the test 

 are rather longer and cylindrical or club-shaped. This species occurs at the Bonin 

 Islands, adhering to the perpendicular faces of rocks exposed to the ocean swells. In 

 this genus, as also in Heterocentrotus and Echinometra, the outline of the test, viewed 

 from above, is eU 

 liptical. The two 

 species of Hetero- 

 centrotus have im- 

 mense club-shaped 

 or angular spines, 

 frequently twice as 

 long as the trans- 

 verse diameter of 

 the test; certainly 

 the most striking 

 productions in the 

 way of spines to be 

 found in the entire 

 class. These spmes 

 are apparently 

 smooth, but actu- 

 ally finely striated. 

 Those immediately 

 round the mouth 

 are flattened, while 

 on the upper sur- 

 face of the test the 

 secondary spines 

 sometimes form a 

 close pavement. 



The auricles are tall and slender, with a large opening. Both species inhabit the 

 Pacific and Indian Oceans, spreading eastward as far as the Sandwich Islands. 



Fia. 148. — Strongylocentrotus drobachiensis, New England sea-urchin. 



