STEUCTUEE OF THE STAEFISH. 77 



divided into two by a median vertical partition : thi& space 

 is the radiating pei'i-hcemal canal. 



I. In the partition notice the cut sections of one or two 

 small tubes, the radial blood-tubes. 



m. Notice the muscular fibres which connect the various 

 ossicles with each other. 



1 . Note especially a muscle which runs across the angle 

 between the ambulacral plates, and separates the water- 

 tube from the peri-ha3mal tube. 



n. Cut as thin a section as possible through the organs 

 in the angle of the ambulacral furrow, and examine it 

 with a higher power, — one hundred to two hundred and 

 fifty diameters, — noticinof : — 



1. The remains of the amljulacral plates (Fig. 33, a m) 

 arching over the ambulacral furrow. 



2. The cross section of the radiating water-tube (i' ic) 

 in the angle between the ambulacral plates. If the section 

 passes through one of the small tubes whicli run oif t5 the 

 aml^ulacra, notice that the layer of epithelium which lines 

 the large tube also lines the small ones. 



3. The muscular band (m) which runs from one ambu- 

 lacral plate to the opposite one underneath the water- 

 tube. 



4. Underneath this muscle notice a space (c) divided by 

 a vertical partition (k) into two. This space is the radial 

 peri-hmmal tube. Notice that the layer of epithelium 

 which lines it is very thin above and on the partition, but 

 thickened below, so as to fonn a pair of opaque granular 

 pads (?). As this structure runs along the whole length 

 of the ray, it is actually not a pad, but a long flattened 

 band. According to some investigators, it is to be re- 

 garded as constituting the nerve-fibre of the ray ; and if 

 this is the true view, it will be seen that there are a pair 

 of radiating nerve-fibres for each ray. 



