448 ECHINODERMATA ASTEROIDEA chap. 



stance of the body-wall, but have a well-defined wall of flattened 

 cells. They are termed, for reasons which will be explained 

 subsequently, perihaemal canals, and tliey open into a circular 

 canal called the " outer periliaennd ring," situated just beneath the 

 water - vascular ring - canal (Fig. 192, perih). These canals 

 originate as outgrowths from the coelom. From their upper 

 walls are developed tlie muscles which connect the pairs of 

 ambulacral ossicles and close the groove, and also those which 

 connect each ossicle with its successor and predecessor and help 

 to elevate or depress the tip of the arm. 



In most of the higher animals the processes of many of the 

 ganglion-cells are connected together in bundles called " motor 

 nerves," which can be traced into contact with the muscles, and 

 thus the path along which the stimulus travels in order to evoke 

 movement can clearly be seen. No such well-defined nerves can 

 be made out in the case of the Starfish, and it is therefore 

 interesting when exceptionally the paths along which stimuli 

 travel to the muscles can be traced. This can be done in the 

 case of the muscles mentioned above. Whereas they originate 

 from the dorsal walls of the perihaemal canals, ganglion-cells 

 develop from the ventral walls of these canals, which are in close 

 contact with the nerve-cord, so that the nervous system of the 

 Starfish is partly ectodermic and partly coelomic in origin. 

 Stimuli reaching the ectodermic ganglion-cells are transmitted 

 by them to the nervous part of the wall of the perihaemal canal 

 and from that to the muscular portion of the same layer of cells. 



Besides the radial perihaemal canals and their connecting 

 outer perihaemal ring there are several other tubular extensions 

 of the coelom found in the body-wall. These are : — 



(1) The " inner perihaemal canal," a circular canal in close 

 contact with the inner side of the outer perihaemal canal 

 (Fig. 19 2, ax^). 



(2) The " axial sinus " (ax) a wide vertical canal embedded in 

 the body-wall outside the stone-canal. This canal opens into the 

 inner perihaemal canal below ; above it opens into several of 

 the pore-canals and into the stone-canal. The separation of the 

 axial sinus from the rest of the coelom is tlie remains of a feebly 

 marked metamerism in the larva. 



(3) The " madreporic vesicle," a closed sac embedded in the 

 dorsal body-wall just under the madreporite. This sac by its 



