Echinoderma. 



125. 



The water-vascular system has no connection with the true blood- 

 vascular system; so that the Echinoderms possess two separate- 

 sets of vessels containing fluid. In the hlood-vascular system 

 there is a circular vessel round the mouth, from which arise numerous 

 branches, amongst them, the radial vessels. In Starfish and Ophiurids, 

 there is a second circular vessel, lying farther from the mouth, and 

 connected with the former by a vascular plexus. There is no heart. 



The alimentary canal differs considerably in the various 

 groups. It may be noticed here, that whilst the mouth is always 

 situated at one pole, the anus (which is usually present) lies in one of 

 the inter-radii, although in some forms it is quite near the aboral pole. 



Special respiratory organs are usually little developed, or 

 entirely absent. They are of various forms : " respiratory trees " in 

 Holothuria, dermal-branchiee in Starfish, and tufted gills at the- 

 mouth of Sea-urchins; these will be considered in greater detail 

 under the different groups. 



Excretory organs are as yet unkno-vm in this group. A glandular 

 organ lying along the stone-canal, -wliicli was formerly regarded as a heart, has 

 more recently been considei-ed to be excretory, but in reality it appears to be a 

 " lymphatic " organ, in which the blood corpuscles are developed. 



¥ig. 78. 



Fig. 78 and 79. Diagram- 

 matic longitudinal section 

 of a Star-fisli and of a Sea- 

 urchin, pass'ng through a 

 radins on the right, an inter- 

 radius on the left, a anus, 

 i gut, fc body-wall, I cEecum 

 of the gut, m mouth, ma 

 madreporite, n radial nerve, 

 o in Fig. 78 eye, in Fig. 79 

 sensory spot, p ampulla, r- 

 radial canal, s stone canal, 

 sk skeletal plate. The Polian 

 vesicles, eio. are left out. — 

 Orig. 



The nervous system in all Echinoderms consists of a nerve- 

 ring round the mouth, from which nerve cords pass off to the radii. 

 In Starfish and Crinoids both the ring and the radial nerve cords lie- 



