244 



ZOOLOGY 



'"Jt'f- 



amount of nutritive matter. Traces of thin-walled vessels 

 occur; one set accompanies the water-vascular system and 

 a complicated set of capillaries lies in the intestines. No 

 " pulsating " heart is known; hence the fluid must pass only 



very slowly tlirough 

 these spaces. 



Organs of ex- 

 cretion are not 

 present, probably 

 because the dis- 

 solved products of 

 decomposition 

 readily pass out 

 through the tube- 

 feet and the skin, 

 which are bathed 



Fig. 234. — View of internal organs of a sea-urchin 

 after removal of the oral half of the body wall. 

 The food canal (aK.) is shown entire, beginning at Ijy constantly re 

 the lantern {lard.) and passing along the a?sophagus 

 (tF.S'.) and intestine {ali. ali.) to the rectum (red.). 

 Parts of five ovaries (ov.) are seen arranged around 

 the anus. The blood vessels are represented Ijy a 

 circular vessel around the mouth (or. r. ves.), one 

 around the anus (ah. r. ves.), and a vessel on the in- 

 testine (int. DCS.). The siphon is shown at siph. 

 From Parker and Haswell. 



newed sea-water. 



Reproductive 

 Organs . — The 

 sexes are usually 

 separate in echino- 

 derms. The eggs or sperm-cells are usually formed in the 

 walls of pouches opening interradially ; in tj^pical sea- 

 urchins, at apertures in five plates lying around the anus. 

 Eggs are poured out and stream down among the spines 

 and often develop there under their protection. 



Musculature. — Owing to the rigid outer skeleton, a gen- 

 eral body musculature is nearly wanting except in holothu- 

 rians. Muscles move the jaws and the spines of sea-urchins, 

 and these are frequently very powerful. 



