292 



THE STARFISH. PHYLUM ECHI NODERM ATA 



- r w. V. 



hollow of the tube-foot is prolonged inwards into a bulb called 

 the ampulla, which projects into the coelom of the arm. The 



transverse canal joins it just below 

 the ampulla, by a valved opening 

 which prevents fluid from flowing into 

 the radial canal, so that, when (by a 

 circular muscle layer) the ampulla con- 

 tracts, the fluid in it stretches out the 

 tube-foot. Muscles in the foot direct it 

 against the ground, by cupping the 

 sucker cause it to adhere, and then 

 shorten the foot, so that it tends to 

 draw the body forward. Owing to the 

 shortening of the foot the fluid passes 

 . ,. , ,^ back to the ampulla. The pressure of 



J. z\i. — A diagram of the ^ . -, . , , 1 . 



water vascular system of a the fluid m the water vascular system 

 s*^""^^^- is regulated by gain, and perhaps 



'''''^•;J:^1^Zt^'^SS^r^^, also by loss, of water through the 



stone canal; t.f.v., vessel of tube rnQrlrpr»nrif P 

 foot ; w.v.r., water vascular ring. liidUicpuiiLC. 



Fig. 217. — 1 

 water vi 

 starfish. 



0S5. 



Fig. 2i». — Diagram of a transverse section of the arm of a starfish. 



a6.musc., Muscle which straightens the arm; ad.oss., adambulacral ossicle; ad.sp., adambulacral spine; 

 atnb.oss., arabulacral ossicle ; amp., ampulla of tube foot ; ep., epidermis ; g., gill ; muse, muscle which 

 opens the ambulacral groove; oss., ossicles; pb.s., peribranchial sinus; />.c., perivisceral cavity; ped. 

 and ped.^ pedicellariae ; py.c, pyloric caecum ; r.b.v., radial ' blood vessel ' ; r.n., radial nerve ; r.ph., 

 radial perihaemal vessel ; r.w.v., radial water vessel ; sp., spine ; t.f., tube-foot. 



