ANATOMY AND DEVELOPMENT OF THE ECHINODERMS I05 



The blood-vascular system is eveiywJicrc totally unconnected with the 

 water-vascular system. 



The water-vascular system, whose real disposition it is of great 

 importance to understand, with reference to embryonic states, Ues, it 

 has been said, superficial to the blood-vascular system. It forms a 

 ring, which lies close to the integument of the mouth in the Ophiurida; 

 and Asteridse, surrounds the oesophagus at the base of the lantern in 

 Echinidse, and encircles it beneath and at some distance from the 

 calcareous ring in the Holothuriadae. 



From this ring a series of vesicles, varying in number from four 

 {Ophiura) to a hundred {Cladolabes peruanus), depend. These are the 

 Polian vesicles ; they open into the water-vascular ring, and appear to 

 be in some way connected with the distribution of fluid through the 

 water-vascular system. 



Connected also with the circular water-vascular ring is the famous 

 sand-canal, of which one or more are found in all the families 

 enumerated. In most there is only one sand-canal, but in some 

 Asterida; there are several, and in Synapta serpentina there are a 

 great number. 



The sand-canal is a membranous tube having calcareous particles 

 imbedded in its parietes, which are sometimes (Holothuriadae) pierced 

 by distinct apertures. 



Now the extremity of the sand-canal may be either adherent to 

 some part of the parietes of the animal, as in Ophiuridse, Asterida;, 

 Echinidae, or it may hang loose in the abdominal cavity, as in the 

 Holothuriadae. In the former case the spot to which it adheres is 

 either entire {Ophiwa), or perforated by many apertures which com- 

 municate with the interior of the canal (Asteridaj, EchinidE), in which 

 case it forms the " madreporic plate." 



But in all cases it is important to recollect that the sand-canal is 

 nothing more than a part of the water-vascular system in which a 

 calcareous deposit has taken place. 



Besides all these appendages the circular water-vessel is connected 

 with five vessels, the water-canals, which supply the tentacles and feet 

 and run down the sides of the body in the ambulacral spaces. 



The nervous ring is formed by a simple cord without ganglionic 

 enlargements, encircling the oesophagus superficial to the water- 

 vascular ring, and giving off five cords which run with, but superficial 

 to, the water-vascular canals in the ambulacral spaces. 



The position of the water-vascular canals and of the nervous cords 

 is apparently different in the Asteridae from what it is in the Echinidae 

 inasmuch as in the former these organs are outside the bony skeleton 



