426 



COMPARATIVE ANATOMY 



CHAP. 



Polian vesicles and, besides (an altogether unique condition), six to fifteen long thin 

 accessory vessels in each interradius, which are hollow and end blindly ; these 

 encircle the intestine and in sexually mature animals penetrate between the genital 

 organs. The walls of these vessels, which are filled with blood and lymph corpuscles, 

 and communicate with the circular canal, consist, from without inwards of: (1) the 



FIG. 364. A portion of the disc of Ophiactis virens in horizontal section, somewhat diagram- 

 matic (after Cu^not). 1, Oral tentacles ; 2, tooth ; 3, circular canal ; 5, section of the stomachal 

 sac ; 6, Polian vesicles ; 7, accessory vessels of the circular canal ; S, stone canal. 



endothelium of the body cavity ; (2) a thin layer of connective tissue ; (3) the inner 

 epithelium. This altogether peculiar development of the water vascular system in 

 Ophiactis virens is considered to be connected with the absence of bursfe which serve 

 for respiration, Ophiactis standing alone among the Ophiuroidea in having no such 

 structures. This peculiar development of the water vascular system is said to be a 

 supplementary means of respiration. 



5. Crinoidea. The circular canal which surrounds the mouth has 

 here no accessory structures except the stone canals. It is provided 

 with longitudinal muscle fibres, which are connected with the epithelial 

 cells (epithelial muscle cells). As in the radial canals, muscle cells 

 also occur transversely traversing the lumen of the canal. The 

 circular canal gives off canals direct to the five groups of tentacles 

 which surround the mouth. 



C. The Radial Canals, the Canals of the Tentacles and Tube-feet ; 

 the Tentacle and Tube-feet Ampullae. 



1. Holothurioidea. The Holothurioidea fall into two very 

 distinct groups, the Si/napt/ihc being- distinguished from all 

 other members of the class by the fact that, in adults, neither 

 tube-feet, tube -feet canals, ampullse, nor any traces of radial 

 vessels are found. The Synaptidce (Pamdinopoda) have only oral 



