ECHINODERMATA . 249 



that of the second or ascending hmb in the left dorsal inter- 

 radius, and that of the third or second descending limb in the 

 right ventral interradius. The pharynx is followed by a 

 stomach with muscular walls ; the intestine forms the longest 

 portion of the alimentary canal ; the posterior end of the rectum, 

 or the cloaca, is rhythmically contractile, and takes in and sends 

 out sea water ; special muscles run between it and the body- 

 wall. 



Certain appendages known as respiratory trees open into the 

 cloaca. These are sometimes regarded as homologous with the 

 two interradial caeca which open into the rectum of Asterids. 

 They are branched structures, usually two in number; each 

 terminal ramification opens by a fine tube into the coelom, and 

 they doubtless serve to introduce sea water into that space. 

 Their function is probably in part respiratory, and they re- 

 semble in structure the similar organs in the armed Gephyrea. 



In some species some of the basal ramifications of the 

 respiratory trees are modified into the so-called Cuvierian 

 organs. In these the peritoneal covering becomes glandular, 

 and when the animals are irritated they are discharged into 

 the water through holes torn at their base in the cloacal wall, 

 and swell up into long tenacious elastic threads, which serve 

 the purpose of entangling their enemies. At times, when much 

 disturbed, the Holothuroidea wiU throw out their whole intes- 

 tine through the anus ; but it is probably regenerated. 



The water-vascular system consists of an oral ring, which 

 gives off five radial vessels, these run at first upwards and out- 

 wards, and give off branches to the oral tentacles ; the tentacles 

 may increase in number with age. The tentacles assist in the pro- 

 cess of feeding, either shovelling in mud or sand — in which case 



11. Anterior part of alimentary canal. 19. Generative organs. 



12', 12", 12'". The three limbs of aliment- 20. Opening of generative duct, 



ary canal. 21. Circular muscles in body-wall. 



13. Cloaca. 22. Bight dorsal muscle. 



14. Cloacal opening with five teeth. 23. Eight ventral muscle. 



15. Radiating muscles of cloaca. 24. Median ventral muscle. 



16. Organs of Cuvier. 25. Left ventral muscle. 

 17', 17". Bespiratory trees. 26. Left dorsal muscle. 

 18. Dorsal mesentery with free posterior 



margin. 



The tentacular ampullae are omitted ; the mouth is in the centre of the divided 



tentacles. 



