HOLOTHURIOIDEA 



413 



Colour whitish, the tentacles usually darker, of a violet tint 

 Grows to a length of ca. 20 cm. 



Biology probably as in the pre- 

 ceding species. Development un- 

 known. 



In British seas this species is 

 recorded from Cornwall, the Irish 

 Sea and the Scottish west coast, the 

 Shetland Islands, Firth of Forth, 

 and from the Northumberland coast. 

 Very probably it really occurs along 

 these coasts, but as there has evi- 

 dently been rather much confusion of the two Thyonidium 

 species and Pseudocucumis mixta, the exact localities given are 

 scarcely quite reliable. The species has about the same hori- 

 zontal and vertical distribution as the preceding species, as far 

 as the identifications are to be relied upon. 



Fig. 250. — Deposits from 

 introvert of Thyonidium 

 commune, x 250. (From 

 an unpublished drawing 

 by Miss Deichmann.) 



II. Family PsoLiDyE^ 



Body usually flattened ; the ventral side forms a thin-walled 

 sole, sharply limited against the dorsal side, which is covered 

 with large, imbricating scales. Mouth and anus dorsal. Tentacles 

 10-15. Tube -feet especially developed along the edge of the 

 ventral sole (viz. in the ventro-lateral radii), forming a continuous 

 circle round the edge, while they are often absent in the mid- 

 ventral radius. On the dorsal side the tube-feet may be present 

 or absent. Mesentery of the posterior intestinal loop in the right 

 ventral interradius. 



Only one genus known from the British and N.E. Atlantic 

 seas. Another genus, Psolidium Ludwig, has been recorded from 

 off Madeira (" Princesse Alice "), namely the species Ps, arcuatum 

 Herouard. It is, however, evident that this species is no true 

 Psolidium, but rather a Cucumaria, and it is mentioned under 

 that genus (p. 396). The true Psolidium has the same general 

 appearance as a Psolus, but it differs from the latter in having 

 tube-feet also on the dorsal side (perforating the scales), while in 

 Psolus tube-feet are present, outside the ventral sole, only on 

 the introvert and, in the shape of small papillae, around the anal 

 opening. 



^ As stated in the Danish work (Echinodermer, Danmarh^s Fauna, 

 p. 239), the author is greatly indebted to Dr. Hj. Ostergren, Gothenburg, 

 for much very important information regarding the characters of the 

 species of Psolus. 



