94 THREE CRUISES OF THE " BLAKE." 



The oldest known sea-urchins belong to the Palaechinidae, a 

 group of palaeozoic echini, having, unlike their modern conge- 

 ners, more than two rows of plates in each zone of the test, and 

 with plates overlapping like the tiles of a roof, so that the test 

 must have possessed considerable flexibility. These urchins 

 were succeeded in mesozoic times by types with a still more flexi- 

 ble test, the coronal plates forming a continuous series from the 

 mouth to the apical system without the usual sharp distinctions 

 of aetinal, coronal, and apical systems. This group is repre- 

 sented in our seas by the Echinothuriae. We may call attention 

 to the characteristic genus Asthenosoma, belonging to the type 



of echini with flexible test 

 and overlapping plates 

 (Fig. 359 a), first described 

 by Grube from a single 

 specimen, and subsequently 

 collected by the " Chal- 



Fig. 359a. — Asthenosoma hystrix. , ,, ~ , ,. , 



lenger. brube did not, 

 however, recognize the great importance of his discovery, and it 

 was not until Thomson and Pourtales dredg-ed these flexible 

 urchins that their affinity to the Echinothuriae of the chalk and 

 to the Palaechinidae became evident. Traces of this overlapping 

 of the coronal plates can still be detected in the most specialized 

 of the recent sea-urchins. 



In one of the hauls taken between Cape Maysi and Jamaica 

 (1,200 fathoms), we obtained the first specimens of Asthenosoma 

 (Fig. 359) I had seen alive. I was much astonished to find 

 them, fully blown up, hemispherical or globular in shape. This 

 was the shape they always took in subsequent hauls, and on 

 several occasions, when they were obtained from comparatively 

 shallow water near the 100-fathom line, they came up alive, 

 and retained their globular outline. The alcoholic specimens I 

 had seen in the "Challenger" collection dredged from deep 

 water were as flat as pocket-handkerchiefs, and were naturally 

 regarded as flat sea-urchins, although of course endowed with 

 great mobility of test. 



Thomson speaks of the vermicular movements passing through 

 the test of Asthenosoma when it assumed on deck what appeared 



