140 



THE YOUNG NATURALIST. 



length than the spines — and being provided 

 with suckers at their ends, the Sea-urchin is 

 enabled to drag itself along. A canal 

 proceeds (Fig. i, s) known as the stone-canal, 

 from the madreporifiorni tubercle, which 

 leads to another canal, forming a ring round 

 the mouth called the circular canal (Fig. i 

 t). This forms the chief centre of the 

 water-vascular system, and gives off five 

 radiating canals, which proceed along the 

 ambulacral areas into the interior of the 

 shell, and give off tubes provided with 

 suckers — the tube feet. At fig. i you 

 will see five organs represented, connected 

 with the circular canal, known as the Polian 

 vesicles, the function of which is very 

 uncertain, 



Fig. I.— Diagram of the ambulacral 

 system of a sea-urchin, m. madreporiform 

 tubercle ; s. stone-canal ; r. central oeso- 

 phagal ring, or circular canal ; p.p. Polian 



I vesicles ; a. a. radiating ambulacral vessels. 

 I Only the bases of four of the radiating 

 I vessels are shown ; and a few of their tube- 

 i feet (t), with their secondary vesicles, or 

 ampulas (v) are shown on one side of one of 

 the radiating canals. (After Nicholson), 



These however are not the only organs of 

 i circulation, there is a blood vascular system 

 j consisting of a contractile cavity, or heart, 

 I which gives off two vessels that form 

 rings round the gullet, and the intestine 

 near the anus respectively. From the 

 latter, or anal ring, five branches proceed to 

 the ambulacral zones, which communicate 

 with five other branches going in the oppo- 

 site direction, and thus convey the blood 

 back again to the heart. 



The mouth is placed at the base of the 

 test and contains a peculiar structure known 

 as Aristotle's lantern. This consists princi- 

 pally of five pieces, somewhat wedge-shaped 

 — the alveoli — each of which forms, so to 

 speak, a socket for a long tooth similar in 

 appearance to the incisor of a Rodent. 

 From this proceeds the oesophagus or gul- 

 let to the stomach, followed by a convoluted 

 intestine winding round the interior of the 

 test and terminating at the apex of the shell 

 in the anus. 



The nervous system is exceedingly simple. 

 It consists mainly of a nervous ring sur- 

 rounding the oesophagus called the circum- 

 oral nerve, which sends off five branches to 

 the ambulacral areas. 



Respiration is effected in all the species, 

 except cidaris, partly by numerous plume- 

 like organs situated round the mouth, and 

 partly by the water-vascular system. 



The generative organs consist of five sac- 

 like glands situated in the spaces between 

 the ambulacral plates, and communicating 

 with the outward world by the pores before 

 referred to on the genital plates. 



Mitton, Stouyporh 



