HOLOTHURIOIDEA 395 



mesenter}^ Usually no Cuvierian organs. Deposits usually more 

 or less irregular, fenestrated plates, sometimes tables. 



The numerous forms of this order feed on plankton or detritus, 

 which they catch by means of their very extensile tentacles. The 

 plankton organisms or the detritus particles stick to the tentacles, 

 which alternately bend down and are put into the mouth, where 

 the food particles are sucked off. 



The order is divided into three families or subfamilies, two 

 of which are represented in the British seas.^ 



Key to the families of Dendrochirota found in the British 

 and the N.E. Atlantic seas. 

 Body usualh^ cylindrical or fusiform, without a well defined ventral 

 sole . . . . . . . .1. Cucmnariidai 



Body more or less flattened, with a sharply defined ventral sole 



II. PsolidcB 



I. Family Cucumariid^ 



Body more or less cylindrical or fusiform, not flattened, the 

 ventral side not forming a thin-walled sole, sharply defined from 

 the dorsal side. Tube-feet either confined to the radii or dis- 

 tributed all over the body. Mesentery of the posterior intestinal 

 loop in the left ventral interradius. 



This large family is divided into two subfamilies, which are 

 thus distinguished : 



Tentacles 10 . . . . . . I. Cucumariince 



„ 15-30 ...... II. Phyllojjhorince 



I. Subfamily Cucumariin.^ 



Ten tentacles, of which the ventral pair usually is smaller than 

 the others, sometimes quite rudimentary or even absent. 



Three genera of this subfamily are represented in British seas ; 

 a fourth, the deep-sea form, Sphcerothuria Ludwig (Syn. Yp)silo- 

 thuria Perrier), is very likely to occur there also, the species Sj^h. 

 talismani (Perr.) (Syn. Y2)silothuria talismani Perr.) ^ being known 

 from the Bay of Biscay to Senegal, 1123-2330 m. (" Talisman ") ; 

 also taken in the North Atlantic by the " Ingolf." 



^ The third family, Rhopalodinidce, contains only the peculiar genus 

 Rhopalodina, from tropical W. Africa. R. Perrier refers to this family also 

 the deep-sea form Ypsilothuria, which seems unacceptable. 



^ Another species, Ypsilothuria attenuata Perr., from off the Cape Verdes, 

 2330 m. (" Talisman "), is so incompletely known that it is scarcely recog- 

 nisable. It is very probably identical with Sph. talismani. 



