CHAP. XIX 



HOLOTHUROIDEA HABITS 



561 





early forms, as, for instance, Bothriocidcu'is, 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 tlie 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 ]5ritish waters. 



One of the best-known of the British 



-A 



species is Holothuria nigra (Fig. 254), 



commonly known as the " Cotton- 

 spinner " ; and this we shall take 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 l)rown 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 oljserved alive 

 and in its natural surroundings, a 

 ring of twenty large tentacles can be 

 seen surrounding the mouth. These 

 buccal tentacles are in every respect 

 comparable with the buccal tube-feet 

 of Ophiuroidea and Spatangoidea, auc 

 in shovelling the muddy substratum 

 into the mouth. 



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

 VOL. I 2 



V 



Fk;. 254.- 

 Buccal 

 X i. 



tentacle 



lik 



them, are employed 

 lich tlie animal lies 



