STRUCTURE OF THE STARFISH. 77 



divided into two by a median vertical partition : this space 

 is the radiating peri-haemal 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-bsemal 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, noticing : 



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

 arching over the ambulacral furrow. 



2. The cross section of the radiating water-tube (r w) 

 in the angle between the ambulacral plates. If the section 

 passes through one of the small tubes which run off to the 

 ambulacra, 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-haemal tube. Notice that the layer of epithelium 

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

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

 pads (I}. 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. 



