ECHINOIDEA 



281 



2. Arseosoma IVIrtsn. 

 (S^Ti. Calveria Wyv. Thomson.) 



Test very flexible ; no sharp Umit between oral and aboral 

 side ; the aspect of the two sides not very different. Ambulacral 

 plates consisting of a large primary plate which is simple, not 

 divided into an inner and an outer plate, and two very 

 small plates lying between the larger plates, far from the edge 

 (Fig. 156, 1). Tube-feet in three dense series. Primary spines 

 on oral side curved, ending in a small white hoof. Tridentate 

 pedicellarise with the valves more or less involute (Fig. 158) ; 

 besides the tridentate and triphyllous pedicellarise a third sort, 

 with four valves, the " tetradactylous " form (Fig. 160), may 

 occur. 



Two species are known from the British seas (and the N.E. 

 Atlantic). 



Key to the species of Arceosoma Icnown from the British 

 {and European) seas. 



Colour of test bright red ; terminal part of large, involuted pedi- 

 cellariae with more or less irregular, never deeply sinuate, edge 

 (Fig. 158, 2). No tetradactylous pedicellarise 



1. A. hystrix (Wyv. Thoms.) 



Colour of test brownish or violet ; terminal part of large involuted 

 pedicellarise with deeply sinuate edge (Fig. 158, 3) ; tetradac- 

 tylous pedicellariae may occur 2. A. fenestratum (Wyv. Thoms.) 



Fig. 158. — Valves of tridentate pedicellariae of Hygrosoma Petersii (1), 

 ArcEosoma hystrix (2), and Arceosoma fenestratum (3), 1 x 65; 2-3 x 75. 

 (After Th. Mortensen, " Ingolf'' Echin., i.) 



