viir EGHINODEEM ATA— RESPIRATORY ORGANS 487 



A. The (inner) Respiratory Trees of the Holothurioidea (Figs. 371 

 and 383, pp. 451 and 477). 



These organs, which are known as water lungs or respiratory 

 trees, occur as two tree-like delicate- walled branched canals or tubes, 

 which lie to the right and left in the body cavity, their principal 

 trunks opening posteriorly into the anterior part of the cloaca. They 

 open either separately or through a common terminal portion. The 

 last branches of the respiratory trees end in vesicular widenings, 

 similar " ampullae " being found also along the branches themselves. 

 When well developed, the respiratory trees reach far forward into the 

 body cavity, being attached at various points by muscle fibres or 

 filaments of connective tissue to adjacent organs, i.e. to the body wall, 

 the alimentary canal, the pharynx, and the mesenteries. In many 

 Aspidochiroice the left respiratory tree is associated with the rete mira- 

 bile of the blood vascular system in the way described on p. 452. 

 The delicate wall of the organ consists of an inner ciliated epithelium, 

 a thin layer of connective tissue, a muscle layer (in which an inner 

 layer of longitudinal fibres and an outer layer of circular fibres can 

 be more or less distinctly made out), and, finally, of the ciliated 

 endothelium of the body cavity. 



There can be no doubt that the respiratory trees actually function 

 as respiratory organs. At regular intervals water flows into them 

 from the cloaca, and is from time to time expelled through the anus, 

 discoloured by the admixture of faecal masses. 



Eespiratory trees are wanting in all Pamdinopoda {Synaptidce), 

 the Pelagolliwiidce, and the Elasipoda, unless, in the last-named family, 

 the diverticulum of the rectum described in the section on the 

 alimentary canal, p. 478, represents a rudimentary lung. 



B. Review of the Respiratory Organs of the Eehlnodermata. 



(«) Holothurioidea Aetinopoda (excluding Elasipoda and Pelago- 

 thuriidce). 



1. The respiratory trees, which open into the cloaca. 



2. The oral tentacles, and to some extent the delicate-walled 



ambulacral tentacles as well. 



{h) Holothurioidea, Paraetinopoda and Pelagothuriidse. 



The whole of the body wall and the oral tentacles. Respiration 

 is promoted by the circulation of the body fluid, kept up by means of 

 the ciliated urns. 



(c) Echinoldea. 



1. The external gills, as outgrowths of the peripharyngeal sinus, 



p. 442. 



2. The ambulacral feet, especially those on the apical surface of 



