414 ECHINODERMS OF THE BRITISH ISLES 



1, Psolus Oken. 

 (Syn. Cuvieria Peron.) 



Tentacles 10. Tube-feet developed almost exclusively on the 

 ventral sole ; outside the ventral sole tube-feet are found only 

 on the soft-skinned introvert and, in the shape of small papillae, 

 around the anal opening, while the w(hole dorsal side is otherwise 

 devoid of tube-feet. Body outside the ventral sole covered with 

 thick, mostly imbricating scales, which usually carry small, 

 rounded grains on their surface. In the ventral sole only 

 scattered microscopical plates are found. 



The Psolids live attached to stones, shells, and other hard 

 objects, their ventral sole acting as a powerful sucking disk. In 

 Psolus phantapus this, however, holds good only for young speci- 

 mens. The larger specimens of this species live free in the 

 bottom, with only the anterior and posterior ends protruding 

 above the ground. 



Three species of this genus are known from the British seas, 

 but three, possibly four more may be expected to occur there 

 also, namely, Psolus nummularis R. Perrier, found at 31° 31' N., 

 12° 47' W., 1050 m. (" Talisman") ; Ps. Pourtalesii Theel, found 

 south of Iceland (" IngoLf "),i known elsewhere from the West 

 Indies ("Blake"); and Ps. tesselatus Koehler, Bay of Biscay, 

 1700 m. (" Caudan " ; only one specimen known). A fourth 

 species, Ps. operculatus Pourtales, has been recorded (Ludwig) 

 from off Spitzbergen ; if the identification be correct, this species, 

 which is elsewhere known from off Florida, must be widely 

 distributed over the North Atlantic. 



Key to the species of Psolus known from or likely 

 to occur in the British seas.~ 



1. Body not flattened ; posterior end produced into a long, tail- 

 like prolongation ; mid-ventral radius of the sole with tube-feet 

 throughout its whole length. Scales rather small ; no large 

 oral valves . . . . .1. Ps. phantapus (Strsf.) 



Body usually flattened, the posterior end not produced into a 

 tail-like prolongation ; mid-ventral radius of sole with tube- 

 feet only at the ends or entirely without tube-feet. Scales 

 rather large ........ 2 



^ According to information by Miss Deichmann ; not yet published. 



2 The fact that young specimens, especially of Ps. phantapvs, are rather 

 unlike the adult makes it necessary to let them appear twice in the key, 

 which is thus rendered more complicated. 



