168 LOWER INVERTEBRATES. 



brilliant sky-blue, and tlie spines of A. 2mlvinata are flesh-colored, with brownish, 

 purple bands. The species named occurs on the west coast of Central America and 

 Lower Califoi-nia. 



The EcHiNOTiiuRiD^ are a family distinguished, among other peculiarities, by the 

 flexibility of the test. This flexibility results from the arrangement of the plates; 

 which, instead of meeting and uniting at their edges, overlap, are separated by mem- 

 brane, and are thus free to move. Prof. A. Agassiz points out that this character is well 

 developed in Astropyga, which may be considered as a connecting link between the 

 Diade'matidse and Echinothuridse. The latter also present many points of resemblance 

 to the extinct Palseechinidse. All the species have extremely depressed tests, resembling 

 at first sight those of a cake-urchin or Clypeastroid, a group which they also simulate 

 in the comparative shortness and small size of the spines. In structure they are, how- 

 ever, regular sea-urchins with the oral and anal systems on o])])osite sides of the test. 



Fig. 147. — Asthenosoma hystrix. 



The genus Asthenosoma contains six species, and occurs at various depths from ten to 

 fourteen hundred fathoms. Although the test is so depressed in preserved examples, 

 living specimens, even when brought up from the moderate depth of one hundred 

 fathoms, are nearly globular, as if the test had been blown up like a foot ball. In 

 I'/iormosoma, of which seven species are known, there is in life a great contrast 

 between the flattened oral side, and the high and globose anal aspect of the test. 

 I", luculentum is perhaps the most Striking species of the group. The test is of a 

 beautiful light violet, foiming a brilliant contrast to the white lines, indicating the 

 sutures of the coronal plates, to the comparatively long, smooth, shining, primary 

 spines, and to the silvery white, thick, hoof-like tii)s that terminate many of the primary 

 spines on the oral surface of the test. 



The structure of the jjlates upon the area round the mouth, which remains flexible 

 in all echini, is in this family so similar to that of the plates of the test itself, as to 

 suggest that they are primarily of similar nature. 



