Introduction to Animal Morphology. 145 



moves from anal to oral ring, and no part of the lining" 

 of this system is ciliated. 



Respiration is thus superficial, but in some there 

 are five peristomial, branched gills, with hollow stems 

 communicating with the body cavity, into which they 

 may admit the sea water ; they are ciliated within 

 and without, extra vascular, and sometimes spiculi- 

 gerous. 



The violet or green circum-oral nerv^e ring is close 

 to the teeth, covered with ganglion cells : it is irre- 

 gular in Spatangoids ; in the others, it sends through 

 each auricular loop an ambulacral nerve, or pair of 

 nerves, which runs between the two series of ambu- 

 lacra, and ends in the ocellus.* Two lateral threads 

 pass to the intestine, and several to the jaw muscles. 

 The ambulacral nerves thicken to the centre of their 

 course. The sphaeridia of Loven, which may be 

 counted as sense organs, are globular or button-like 

 bodies on stalks, placed along the ambulacral line and 

 on the peristomial plates. They are absent in Cida- 

 ridae, sunk in dome-like hollows in Clypeastridae.f 



The genital glands lie one in each inter-ambulacrum. 

 The testes are white, the ovaries reddish or yellow, 

 both lanceolate and plicated : they are four, and un- 

 symmetrical in most of the eccentric forms. The 

 ducts traverse the pores of the genital plates, and the 

 products are slowly emptied. The eggs are small, 

 10-12 in each grape-like bundle of the ovary. Half- 

 an-hour after impregnation, yelk rotation begins ; in 

 3-4 hours fission occurs ; and in eight hours each egg is 



* The spines round the ocellus are called " eyelids." 

 t There may be one or two sphaeridia in each peristomial plate. They 

 move slowly in Brissopsis, 



L 



