ASTERIAS NERVOUS SYSTEM 



445 



isolated sense-cells are found, and it becomes impossible to speak 

 of a sense-organ. 



A prolongation of the radial nerve -cord extends outwards 

 along one side of each tube-foot. This is often spoken of as 

 the " pedal nerve/' but the term nerve is properly retained for 

 a mere bundle of axons such as we find in the higher animals, 

 whereas the structure referred to contains the bodies of nerve- 

 cells as well as their outgrowths or cell-fibres and is therefore a 

 prolongation of the nerve-cord. 



S P . 



iV 



Fig. 192. Diagrammatic longitudinal section through a young Asteroid passing through 

 the tip of one arm and the middle of the opposite interradius. This diagram is 

 generalised from a section of Asterina gibbosa. ab, Aboral sinus ; ax, axial sinus ; 

 ax 1 , basal extension of axial sinus forming the inner perihaemal ring-canal ; br, 

 branchia = gill = papula ; g.r, genital rachis ; mp, madreporite ; musc.tr, muscle 

 uniting a pair of ambulacral ossicles; nerv.circ, nerve -ring; n.r, radial nerve- 

 cord ; oc, eye-pit ; oss, ossicles in skin ; p.br, peribranchial sinus ; p.c, pore canal ; 

 perih (on the right), perihaemal radial canal, (on the left), outer perihaemal ring- 

 canal ; py, pyloric caecum ; red, rectum ; rect.caec, rectal caeca ; sp, spines ; st.c, 

 stone-canal ; t, median tentacle terminating radial canal ; w.v.r, water- vascular 

 radial canal. The genital stolon (not marked by a reference line) is seen as an 

 irregular band accompanying the stone-canal, its upper end projects into a small 

 closed sac, also unmarked, which is the right hydrocoele or madreporic vesicle. 



At the base of the terminal tentacle the radial nerve-cord 

 ends in a cushion. This cushion is called the " eye," for it is 

 beset with a large number of cup -shaped pockets of the 

 ectoderm. Each pocket is lined partly by cells containing a 

 bright orange pigment and partly by visual cells each of which 

 ends in a small clear rod projecting into the cavity of the pit 

 (Fig. 193, A, vis.r). The pit is apparently closed by a thin sheet 

 of cuticle secreted by the most superficial cells. 



An exposed nervous system and simple sense-organs such as 

 the Starfish possesses lend themselves admirably to the purposes 



