STRUCTURE OF THE STARFISH. 



81 



similar in structure to the thickening in the ambulacra! 

 furrow. 



FIG. 35. 



FIG. 35. Diagram of a vertical longitudinal section through the 

 mouth pentagon in the axis of a ray, magnified about one hundred and 

 fifty diameters. (Copied with slight changes from H. Ludwig; Zeit. f. 

 Wiss. Zool. xxx. Taf. VI., Fig. 16.) 



a m. The first ambulacra! plate, a m'. The second ambulacral plate. 

 c w. Circum-oral water-tube. r w. Radiating water-tube. m and m'. 

 Muscles which join the pair of plates, o m. m 1 '. Muscles which join the 

 pair of plates a m'. a. Inner channel of circum-oral peri-hsemal vessel. 

 c. Outer channel of same, at the origin of the radiating peri-haemal ves- 

 sel, d. Radiating blood-vessel, k. The partition which supports it 

 along the middle line of the ray. e. Partitions which support the lateral 

 blood-vessels. b,f, y, h, and p. As in Fig. 34. 



