178 



PHYLUM ECHINODERMATA. 



The coelom. The relations and development of the coelom 

 have been more fully elucidated in Asteroids than in any other 

 class of Echinodermata. It represents the following parts : * 

 (1) the general or peri visceral body-cavity, (2) the axial sinus 

 and inner circumoral perihaemal ring, (3) the so-called outer 

 circumoral perihaemal ring with its radial prolongations between 



15 



21 



FIG. 129. Diagrammatic view of a transverse section through the arm of an Asteroid (iron 

 Lang). 1 trunks of the deep oral nervous system (Lange's nerves) ; 2 rajiialwater-vascular 

 trunk ; 3 tissue of so-called radial blood-vessel ; 4 radial nerve of the superficial oral 

 system ; 5 radial perihaemal canal ; 6 and 7 branches of the same to the walls of the tube- 

 feet ; 8 stalked "pediWllaria*; 9 spine ; 10 genital aperture ; 11 papula with contained body 

 cavity 13; 12 sessile pedicellaria ; 14 hepatic caecum ; 15 peribranchial cavity ; 16 supra- 

 marginal ; 17 inframarginal ; 18 adambulacral plate ; 19 marginal canal communicating 

 at 20 with the perivisceral coelom ; 2 1 peritoneal epithelium ; 22 genital sinus ; 23 gonad ; 

 24 mesenteries of hepatic diverticula ; 25 tube connecting tube-foot with 26 ampulla ; 

 27 cavity of tube-foot ; 28 upper and lower transverse muscles connecting the ambulacral 

 plates ; 29 branches of the deep oral nerve trunks ; 30 ambulacral plates ; ^perivisceral 

 cavity ; 32 longitudinal muscle ; 33 apical nervous system. 



the ambulacral nerves and the radial water-vascular trunks, 

 (4) the aboral sinus surrounding the generative rachis and the 

 sinuses in the walls of the gonads, (5) the water- vascular system, 

 and (6) the gonads. 



The general body-cavity which is derived from the right and 



* For the general relations and development of these structures the 

 reader is referred to p. 126 et seq. and p. 142 et seq. 



