94 THREE CRUISES OF THE “BLAKE.” 
The oldest known sea-urchins belong to the Palechinide, a 
group of palseozoic 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 actinal, coronal, and apical systems. This group is repre- 
sented in our seas by the Echinothurie. 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- 
lenger.” Grube did not, 
however, recognize the great importance of his discovery, and it 
was not until Thomson and Pourtalés dredged these flexible 
urchins that their affinity to the Echinothurie of the chalk and 
to the Palechinide 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 
Fig. 359 a. — Asthenosoma hystrix. 
