XIV] HOLOTHUROIDEA 357 



and so the small tube-feet present in such multitudes in the 

 Clypeastroidea are absent. Besides these spin es they possess peculiar 

 lines of very small spines covered with cilia, which cause a current 

 to pass over the gill-like tube-feet. Such rows of ciliated spines are 

 termed fascioles. 



IV. HOLOTHUROIDEA. 



The fourth group of the Echinoderms is termed the Holothu- 

 roidea or Sea-cucumbers, and consists of animals of a more or less 

 sausage-shaped form, with the mouth at one end and the large anus 

 at the other. 



These animals have undergone the same essential modification 

 as the Sea-urchins, the arms having been re-absorbed into the body 

 so that the radial tubes run down the side of the body and end 

 near the anus. The nervous system also is situated beneath the 

 surface, the ambulacral groove being represented by the epineural 

 canal. The skin, as in Echinoidea, retains its well-marked ecto- 

 derm with the nervous layer at its base. 



They are however distinguished by some most marked charac- 

 teristics: 1. The skeleton has almost entirely disappeared, being 

 represented only by grains and prickles of various shapes completely 

 buried in the skin. 2. The muscular system of the body-wall is 

 most powerfully developed: there is a pair of strong longitudinal 

 muscles running inside each radial water-tube, and transverse 

 muscles run across each interradius. 3. The buccal tube-feet . 

 termed "tentacles" are highly modified and are the means by 

 which the animal feeds itself. 4. The anus is wide and the 

 concluding portion of the intestine termed the cloaca is strongly 

 muscular, and it is used as a breathing organ, water being sucked 

 in at the anus and thrown out again. 5. The stone-canal does not 

 reach the exterior, but terminates in a sieve plate hanging down 

 into the interior of the body. 



The breathing by means of the anus is carried out by certain 

 organs called gill -trees. These are two great branched tree-like 

 outgrowths of the hinder part of the intestine, reaching right 

 through the body-cavity to near the mouth (Fig. 168). Water is 

 taken in by the anus and forced up into the finest branches and no 

 doubt diffuses through their walls into the body-cavity under the 

 pressure set up by the contraction of the muscles of the anus. Hence 

 it is that the animal is able to dispense with an external madreporite, 



