44 



FIR8T LESSONS IN ZOOLOGY. 



We are now ready to examine the internal organs and to study their 

 relations to one another and to the body-walls. The nervous system 

 may be seen without dissection. By closely examining the mouth a 

 pentagonal ring is seen surroimding it, each angle slightly enlarging 



Fig. 43. — Longitudinal section through the body and one arm of Asterias vtil- 

 garis. ni. mouth ; Sj stomach ; i, lobe of stomach extending into the arm ; a, 

 anus; nr^ nervous ring; n, radial nerve; vr. water-vascular ring, sending a. 

 radial vessel (V) into the arm; njd, madreporic plate; f, stone-canal; /t. hsemal 

 canal; ou, oviduct; o, ovary; am, ampullse, the ambulacral feet projecting 

 below; b, cceca or liver. 



and sending ofE a nervous cord to the eye at the end of the ray. It 

 may be discovered by pressing apart the ambulacral feet along the 

 median line of each arm. Fine nerves are sent off to each sucker, 



Fio. 44.— Diagram of the water-system of a starfish, a, madreporic body; h. 

 stone-canal; c, circumoral water-tube; d, water-tubes to the arms; e, am- 

 pullse; /, feet or suckers. 



passing through the opening between the calcareous plates and extend-'* 

 ing to each ampulla, thus controlling the movements of the ambula- 

 cral feet. The skin is also underlaid by a sheet of nervous tissue. 



