THE HOLOTHURIDEA. 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 c^ecal prolongation on the outer side 

 of the calcareous ring. The ambulacral vessels are filled with 

 a fluid containing numerous nucleated cells. 



Contractile vessels, which accompany the intestine, and 

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

 fluid, seem, notwithstanding the difl'erence 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-oesophageal plates already mentioned, 

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

 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.^ 



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 c;T3cal tubuli of which it is composed contain 

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

 maphrodite. In the majority of the Holothuridea^ however, 

 the sexes are distinct. 



In other Holotliurldea^ the skeleton may attain a much 

 greater development, nnd 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 radicd 

 amhidacral vessels. In the higher Ilolothuridea^ each radial 

 ambulacral vessel gives ofi" manv lateral branches; these enter 

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



1 Accordinnc to Greef (" Ueber den Bau der Echinodermen," 3te Mittheilung, 

 Sitzunwsbericlite der Gesellscliaft zu Marburo:, 1872), another canal lies super- 

 ficial to the ambulacral nerve in the Holothu^^id^a^ and represents the ambu- 

 lacral (yroove of the star-fishes, Teuscher, " Beitr",afe zur Anatomie der Echi- 

 nodermen" (t/£;ftai-s"7i? ^e2:/^w^ri/'^, 1876;, however, maintains that this superfi- 

 cial canal is an artificial product. 



