64 INTRODUCTION TO ZOOLOGY chap. 



General Body- The cavity surrounding the alimentary canal 

 cavity and is the body-cavity or " coelom," ill which lie all the 

 its Contents. Q^j^g^ organs of the body. These are found to 

 have a radial symmetry corresponding to the external 

 symmetry, e.g. there is a water-vascular system and a nervous 

 system each including a circum-oral ring and a radial extension 

 into each arm. (These are omitted in Fig. 31.) Each radial 

 nerve is connected with a little red eye-spot lying just below 

 the tip of each arm, and frequently exposed by the upturning 

 of the arm (Fig. 31, e). The eye-spot is a complex structure 

 consisting of from one to two hundred little lenses, below 

 which are pigment cells, the whole being sensitive to light. 



The generative organs are likewise radially arranged, though 

 they alternate with the arms instead of corresponding with 

 them in position. The sexes are distinct ; there are five 

 pairs of ovaries or spermaries in each individual in between 

 the bases of the arms (Fig. 31, g) ; each pair has a separate 

 duct which opens by a pore on the dorsal surface of the body. 

 The eggs are fertilised in the water after having been dis- 

 charged from the ovary. 



. Respiration seems to be chiefly effected by 



' means of little thin-walled outgrowths of the skin, 



which project from the upper surface and the sides of the 



starfish. They can be clearly seen when the starfish is under 



water (Fig. 31, h). The cavity of these is continuous with 



the body-cavity, and the oxygen from the sea-water can diffuse 



through them to the fluid which fills the body-cavity, and 



so pass to all parts of the body. There seems to be no 



definite blood -vascular system, though certain canals and 



spaces that occur were originally wrongly described as such. 



Water- Within the body- cavity will also be found the 



vascular curious water-vascular system on which depends 



System, ^jjg action of the tube-feet, and which is only 



found in the group of the Echinoderms. 



Each tube-foot is seen to pass through the external body- 

 wall and to swell inside into a little vesicle or "ampulla'" 

 (Fig. 31, ^. Each vesicle is connected by a very short tube to 

 a radial canal which extends the whole length of the arm, and 

 the five radial canals unite in a circular ring just above the 

 mouth (Fig. 31, mth). This ring is known as the "ambulacral 

 ring," and it connects with the exterior by a single tube with 



