THE HOLOTHURIDEA. 4G9 



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

 riiio- on tlie 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 tilled 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 diff"erence in their contents, 

 to represent the pseud-hsemal 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 cfecal tubuli of which it is composed contain 

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

 maphrodite. In the majority of the Solothuridea, 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 radial 

 ambulacral vessels. In the higher Ilolothuridea, each radial 

 ambulacral vessel gives ofi" many lateral branches; these enter 

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



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

 Sitzungsberichte der Gesellschaft zu Marbursr, 1872), another canal lies super- 

 ficial to the ambulacral nerve in the Holothuridfa^ and represents the ambu- 

 lacral groove of the star-fishes. Teuscher, " Beitnige zur Anatomic der Echi- 

 nodermen" (.7^/i«?;<tr^(? ;?«fec^rif^, 1876), however, maintams that this superfi- 

 cial canal is an artificial product. 



