Till: llni.nTIlURIDEA. 469 



and the anterior end of the alimentary canal and the nerve- 

 ring on the inner side. As each enters its tentacle, it dilates 

 and sends down a short caecal prolongation on the outer side 

 of the calcareous ring. The ambulacral vessels are tilled with 

 a 11 1. id containing numerous nucleated cells. 



Contractile vessels, which accompany the intestine, and 

 lie on opposite sides of it, tilled with a similar corpusculated 

 fluid, seem, notwithstanding the difference in their contents, 

 to represent the pseud-haemal vessels of the Annelids. These 

 vessels do not extend into the parietes of the body. 



The nervous system consists of a ring which lies superfi- 

 cial to the circular water-vessel, and from which five principal 

 equidistant cords proceed. These pass through the apertures 

 or notches in the circum-cesophageal plates already mentioned, 

 and each proceeds along the middle line of one of the lonffi- 

 tudinal muscular bands, to the opposite extremity of the 



body. 



The ambulacral nerves appear to be hollow ; or perhaps it 

 would be more correct to regard them as thickenings in the 

 wall of a neural canal, as they are in the Asteridea. 1 



The genital gland is single, and opens near the oral end 

 of the body, in the line of the attachment of the mesentery. 

 The branched caecal tubuli of which it is composed contain 

 both ova and spermatozoa, so that the Synaptce are her- 

 maphrodite. In the majority of the Holothuridea, however, 

 the sexes are distinct. 



In other Holothuridea, the skeleton may attain a much 

 greater development, and even take the form of conspicuous 

 overlapping plates (Psolus). Moreover, the circular vessel 

 of the ambulacral system not only gives origin to Polian vesi- 

 cles, madreporic canals, and tentacular vessels, but five canals 

 proceed from it, pass through holes or notches in those cir- 

 cum-oesophageal plates to which the longitudinal muscles are 

 attached, together with the nerves, and run backward, along 

 the centre of the area occupied by these muscles, on the deep 

 or inner side of the longitudinal nerve. These are the ru-H'il 

 ambulacral vessels. In the higher Holothuridea, each radial 

 ambulacra] vessel gives off many lateral branches; these enter 

 contractile processes of the body-wall, which subserve loco- 



1 .Vvoi\lin<_r to Greef (" Ueber den Bau der Echinodermen," 3tc Mittheilung, 

 Bitntngsberionte der Gesellschafl /u Marlmrsr, 1S72), another canal lii's sui>-r- 

 tu-ial t<> tin- ambiilacral nerve in the Holothuridfa, And represents the ambo- 

 lai-ral irionvi- oi' tin- star-tishes. Teuscher, " B.ritivirf y.ur Anatomic -lor Kvhi- 

 nodennen" (Jena >*<}> 7,,it*cl>riff, 1876), however, maintains that this superfi- 

 cial canal is an artificial product. 



