CHAP. XIX 



HOLOTHUROIDEA HABITS 



5 6l 



early forms, as, for instance, Bothriocidaris, Holothuroidea may- 

 have been evolved. The muscular body-wall has indeed been 

 as important a factor in the evolution and differentiation of the 

 Holothuroidea as the muscular arm in that of Ophiuroidea, or 

 the movable spine in the case of Echinoidea. 



There are about 520 species of living Holothuroidea, and of 

 these about twenty-one have been recorded from British waters. 

 One of the best-known of the British 

 species is Holothuria nigra (Fig. 254), 

 commonly known as the " Cotton- 

 spinner " ; and this we shall take t as 

 a type for special description. The 

 animal may attain a length of a foot 

 when fully extended, and has a 

 diameter of from 3 to 4 inches. It 

 is of a very dark brown colour on 

 one side, which in crawling it keeps 

 uppermost, whilst on the lower side 

 it is of a tawny yellow hue. Three 

 of the radii (often termed the "tri- 

 vium ") are situated on the lower 

 surface ; two (termed the " bivium ") 

 on the upper surface. The podia are 

 scattered fairly evenly over the whole 

 surface without reference to the radii ; 

 below they are regular tube-feet pro- 

 vided with suckers, whilst on the 

 upper surface they are pointed ten- 

 tacles, employed only for sensory 

 purposes. 



If the animal be observed alive 

 and in its natural surroundings, a 

 ring of twenty large tentacles can be 

 seen surrounding the mouth. These Fig. 2U.-Hoiothvria nigra, t, 



Buccal tentacle or feeler. 



buccal tentacles are in every respect x j, 



comparable with the buccal tube-feet 



of Ophiuroidea and Spatangoidea, and, like them, are employed 



in shovelling the muddy substratum on which the animal lies 



into the mouth. 



Ludwig employs the term " feeler " for these buccal tentacles, 



vol. 1 



2 



