Angulcw Sea-Cucumher. 149 



logons, not, as they really are, with the masticatory apparatus, the 

 so-called 'Aristotle's Lantern,' of the Eehinoidea, but with the 

 ' auriculae' of these Echinodermata, and with the proximal pair 

 of 'vertebral ossicles' (see p. 144, supra) of Star-fishes. The 

 water- vascular ring surrounds the muscular pharjaix a little 

 way posteriorly to the calcareous ring ; two long air-bladder-like 

 Polian vesicles are seen hanging down from it, and ending freely 

 in the body cavity on the left side ; whilst the madreporic plate 

 and the pedicle upon which it is placed are seen to join it in the 

 medio-dorsal line. The internal position of the madreporic tubercle, 

 characteristic of the Holothurioidea, renders it necessary that the 

 water in the locomotor water- vascular system should be obtained 

 from the fluid in the perivisceral cavity, from which the pores of 

 the madreporic disk are, however, said to be separated by a layer 

 of epithelium, continuous with that lining the perivisceral cavity ; 

 so that the fluid they receive enters them not directly but by 

 diffusion through and by the intermediation of these cells. In 

 immediate relation with the water-vascular ring is seen an annular 

 plexus of pseud-haemal vessels, which represents the circular pseud- 

 haemal sinus of the other Echinodermata. From it two principal 

 vessels pass backwards ; one along the dorsal, the other along the 

 ventral line of the digestive tract, in the substance of the walls 

 of which they are connected with each other by plexuses. Of 

 these two vessels, the ventrally-placed one is always eomj^aratively 

 simple and devoid of any ramifications of importance, though the 

 segment of it in connection with the first descending convolution 

 of the intestine communicates with that in connection with the 

 first ascending, by one or more transverse commissural vessels; 

 w^hilst the dorsal vessel may have a considerable reie mirah'ile 

 developed in connection with it, which in Aspidochirotae comes 

 into relation with the left ' respiratory tree' arising from the cloaca. 

 This rete miraUle may be seen in this specimen in connection with 

 the dorsal vessel in the first segment of the intestine ; it must not, 

 however, be confounded with the reticular muscular mesentery, 

 the very abundant fenestration of which gives it the appearance 

 of a plexus of blood-vessels. The muscular pharynx is succeeded 

 by a muscular stomach, which is much smaller in calibre and a 

 little shorter in length than the portion of the pharynx which 

 intervenes between the commencement of the stomach and the 



