412 ECHINODERMS OF THE BRITISH ISLES 



introvert and the posterior end of the body. Calcareous ring 

 with radials and interradials distinct even in large specimens. 

 Colour in life usually reddish. Grows to a length of ca. 

 12 cms. (or more ?). 



It lives especially on a sandy, muddy bottom, and usually 

 covers itself with shell fragments, etc. It is much eaten by fishes, 



Fig. 249. — Deposits from body wall of Thyonidiuin pellucidum. 



X 145. (From Danmark's Fauna.) 



1. From above. 2. Half side view. 3. Side view. 



especially the cod. Often the fishes bite off the tentacles or the 

 whole anterior end. Development unknown. 



In British seas this species is known from S.W. Ireland to 

 the Shetland Islands, and down to the Northumberland coast on 

 the east side. Elsewhere distributed from the Arctic seas to 

 the Channel, and from Greenland to Florida. Bathymetrical 

 distribution ca. 10-380 m. 



2. Thyonidium commune (Forbes). (Fig. 250.) 



(Syn. Thyonidium, Phyllophorus Drummojidii (Thompson) ^ ; 

 Thyonidium Dilbeni Norman ; Cucumaria communis Forbes ; 

 Thy one Portlocki Forbes.) 



Body rather elongate, cylindrical, tapering towards the ends. 

 Tentacles of each pair of the inner circle distinctly separated. 

 Body wall thick, not transparent even in small specimens. 

 Tube-feet very fine and very numerous, more or less distinctly 

 arranged in series. Deposits completely absent, except in the 

 introvert ; they are small, delicate tables with a high, slender 

 spire (Fig. 250), which may sometimes be reduced, the disk alone 

 being left ; in the tentacles large fenestrated plates. Calcareous 

 ring in larger specimens large, shapeless, while in young speci- 

 mens the radials and interradials are distinct, as in Th. jyellucidum. 



^ The name Drummondii Thompson ought not to be used, as it is quite 

 impossible to say to which species the original description really refers. 

 The form described by Forbes under that name is certainly not a Thyo- 

 nidium; the arrangement of the tube-feet, as shown by his figure (p. 223), 

 rather suggests Pseudocucumis. 



