CH. XIV] HOLOTHUROIDEA 359 



and also to obtain the fluid necessary to keep its tube-feet tense 

 from its own body-cavity. From the water-vascular ring one or 

 more long-stalked Polian vesicles hang down into the body- 

 cavity. t 



The muscular body- wall has a very curious effect on the economy 

 of the animal. When it is irritated it contracts the muscles, and 

 since the fluid in the body-cavity is practically incompressible, the 

 effect is to set up a tremendous pressure. As a result of this the 

 wall of the intestine near the anus tears and a portion or the whole 

 of the intestine is pushed out. The gill-trees are the first to go, 

 and in some species the lower branches of these are covered with a 

 substance which swells up in sea-water into a mass of tough white 

 threads in which the enemies of the animal are entangled. A 

 lobster has been seen to be rendered perfectly helpless as a con- 

 sequence of rashly interfering with, a Sea-cucumber. These special 

 branches of the gill-trees are termed Cuvierian organs. 



A Holothuroid is only temporarily inconvenienced by the loss 

 of its internal organs. After a period of quiescence it is again 

 furnished with the intestine and its appendages. Some species, which 

 are able to pull in the mouth end of the body with the tentacles, 

 when strongly irritated snap off even this, and yet are able to repair 

 the loss. 



The intestine is a simple looped tube which has three limbs. 

 One limb runs down towards the anus, the next turns up again 

 towards the mouth and then bends back into the final limb which 

 goes towards the anus. These limbs are attached by mesenteries 

 to different interradii of the body, the first to that which in the 

 ordinary position of the animal is mid-dorsal, the next to the left 

 ventral, and the third to the right ventral (Fig. 168). 



Accompanying the alimentary canal are so-called dorsal and 

 ventral "vessels" similar to those of the Echinoidea, and there is 

 also a "blood-ring" like that described in the same class. In 

 Holothuroid ea the ventral vessel is close to the alimentary canal 

 but the dorsal vessel is borne on a little ridge projecting from the 

 intestine. The alimentary canal is enswathed by minor branches of 

 the network of which the dorsal and ventral vessels form merely the 

 large trunks. The whole system thus assumes a very complicated 

 appearance, but even here it has been shown that there is no 

 circulation nor even a proper wall to the spaces. The longitudinal 

 vessels indeed often do not appear to communicate with the blood- 

 ring. 





