ECHINODERMS OF THE BRITISH ISLES 



divide the body into two symmetrical halves. There is thus in 

 the echinoderms a peculiar combination of a radiate and a 

 bilaterally symmetrical structure. 



The same arrangement as is found' in the watervascular 

 system is shown by the nervous system and the blood system : 

 around the oesophagus a ring from which issues a main branch 

 to each radius. The nervous system is simple, without ganglia ; 

 it lies in sea-stars and sea-lilies in the epidermis, in the other 

 echinoderms inside the epidermis. In addition to this 

 ambulacral nervous system lying in the epidermis, or at least 

 originating from the epidermis, there are in the echinoderms 



two other nervous systems. One 

 of these lies along the inner 

 side of the ambulacral nervous 

 system, following the latter. The 

 other lies on the dorsal side, 

 under the skin. This dorsal 

 nervous system is in the sea- 

 lilies much more developed than 

 the two other systems, while in 

 the other echinoderms it is only 

 slightly developed. The echino- 

 derms thus have no less than 



Fig. 2.— Diagrammatic section of three nervous systems, 

 an arm of a Sea -star. 

 (From Danmark's Fauna.) 



A, Ampulla; B, Kadial blood-vessel; 

 G, Gill (papula) ; L, Liver ; M, Muscles ; 



Ms Mesentery ; Ni, Ambulacral nervous , , . , , 



stem, in the epidermis ; Nj, Inner nervous ncrvOUS System, lying between 

 stem : No, Dorsal nervous stem ; R, Eadial ,^ ^ i ^i 



water-vessel ; sf, Tube-foot. the nervous System and the 



watervascular system. From 

 the circular vessel that surrounds the oesophagus issue, besides 

 the radial branches, vessels to the intestine and also a vessel 

 w hich accompanies the stone canal ; the latter vessel is in con- 

 nection with a dorsal or aboral haemal ring from which branches 

 pass out to the genital organs. There is no heart and no 

 pulsating vessels. In some sea-cucumbers the intestinal vessel 

 forms a rich plexus of blood-vessels, a rete mirahile. A ^vell- 

 defined system of lacunae or " sinuses " surrounds both the 

 circular and the radial vessels and their side branches. Fig. 2 

 shows the relative position of nervous systems, blood-vessels, 

 and watervascular system. 



Kidneys [excretory organs) are lacking ; the excretory function 

 is undertaken b}^ amoeboid wandering cells, which are either 



The blood vascidar or hcemal 

 system follows immediately on 

 the inside of the ambulacral 



