HOLOTHURIOIDEA 379 



1. Mesothuria Ludwig. 

 (83^1. Allantis Herouard ; Zygothuria Perrier.) 



Body usually cylindrical, slightly attenuating towards both 

 ends, without marginal fringe. Twelve to twenty tentacles. 

 Mouth terminal. Genital organs only on the left side of the 

 dorsal mesentery. Skin thin. Calcareous deposits tables ; no 

 C-shaped spicules. Development unknown. 



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

 and six more may be expected to occur there also, viz. Mesothuria 

 candelabra Herouard, Bay of Biscay, 4870 m. (" Princesse Alice " ; 

 only some pieces of skin have been found) ; M. connedens 

 (Perr.) (Syn. Zygothuria connectens Perr.), off the Canaries, 

 1975-2518 m. (" Talisman ") ; M. expectans Perr., 44° 20' N., 

 19° 31' W., 4255 m. (" Talisman " ; only one specimen known) ; 

 M. maroccana Perr., from off the Moroccan coast and between 

 Spain and the Azores, ca. 2000-3120 m. " Talisman", " Michael 

 Sars ") ; M. Murrayi Theel, var. grandipes Herouard, from 

 between the Azores and Madeira, ca. 1500-2180 m. (".Princesse 

 Alice ") ; M. rugosa Herouard, off the Cape Verdes, 3890 m. 

 ('' Princesse Alice " ; only one specimen known). 



Key to the species of Mesothuria known from or likely 

 to occur in the British seas. 



1. Ventro -lateral ambulacra with no distinct series of larger tube- 



feet 2 



Ventro -lateral ambulacra with one or two distinct series of 

 larger tube -feet ^ . . . . . . . .5 



2. Deposits of 4-radiate type, with spire composed of 4 rods . 3 



3- „ „ „ „ 3 „ . 4 



3. Spire of tables with a crown of several thorns ; tables of tube- 



feet like those of the skin . .1. M. intestinalis (Ascan.) 



Spire of tables with a crown of only 4 thorns ; tables of tube- 



feet reduced, less developed than those of the skin (Fig. 224, 



4-5) 2. M. Verrilli (Theel) 



4. Spire regular, the three component rods equally developed, 



ending in 3-5 thorns each ; the disk regular with 6 peripheral 

 holes (Fig. 224, 1) (" Talisman " Holoth., p. 312, PI. X\T. 

 32-35) ...... M. maroccana Perr. 



Spire irregular, the three component rods unequally developed ; 



^ The position of M. candelabra in one or other of these two groups is 

 uncertain, only the calcareous deposits being known, but judging from 

 these it would seem to be near to M. connectens, and is therefore placed in 

 the same group as this species. M. expectans is possibly only the young of 

 Paroriza Prouhoi. 



