of the Echinoderms. 3 



cular ring round the oesophagus with another vascular ring sur- 

 rounding the anus, from which branches pass to the ovaria, &c. 



Branches are given off from the principal blood-vascular ring 

 towards the ambulacra, and in the Holothuriadse it appears very 

 probable that these branches accompany and indeed inclose the 

 nerves. 



The blood-vascular system is evei'ywhere totally unconnected 

 ivith the wate7'-vascular system. 



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

 importance to understand, with reference to embryonic states, 

 lies, 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 Ophiuridae 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 Holothuriadse. 



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 distri- 

 bution 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 AsteridjE there are several, and in Synapta serpentina 

 there are a great number. 



The sand-canal is a membranous tube having calcareous par- 

 ticles imbedded in its parietes, which are sometimes (Holothu- 

 riadfe) pierced by distinct apertures. 



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

 to some jjart of the parietes of tlie animal, as in Ophiurida3, As- 

 terid?e, Echinidse, or it may hang loose in the abdominal cavity, 

 as in the Holothuriada^. In the former case the spot to which 

 it adheres is either entire {Ophiura), or perforated by many aper- 

 tures which communicate with the interior of the canal (Aste- 

 ridse, Echinida), 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 con- 

 nected with five vessels, the water-canals, which supply the ten- 

 tacles and feet and run down the sides of the body in the ambu- 

 lacral spaces. 



The nervous ring is formed by a simple cord without gan- 

 glionic 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. 



1* 



