252 



PHYLUM ECHINODERMATA. 



round the oesophagus placed aboral of the calcareous ring in 

 the outer wall of the perioesophageal sinus (Fig. 179) ; (2) five 



FIG. 179. Diagram of a longitudinal section through the oral region of a Holothurian. The 

 section passes through a radius on the right side, and through an interradius on the left 

 (from Lang). 1 cutis, 2 ectoderm, 3 oral tentacle cut off, 4 canal of the oral tentacle, 

 5 blood-vessel of the oral tentacle, 6 tentacle nerve, 7 circumoral nerve, 8 oral portion of 

 perioesophageal sinus, 9 mouth, 10 oesophagus, 11 perioesophageal sinus, 12 interradial 

 piece of calcareous ring, 13 circumoral water- vascular vessel, 14 so-called blood- vascular 

 ring, 15 ventral intestinal vessel of the so-called vascular system, 16 epithelium of alimen- 

 tary canal, 17 polian vesicle, 18 ampulla of oral tentacle, 19 peritoneal epithelium, 20 

 circular muscles of body wall, 21 body-cavity, 22 and 26 radial vessel of vascular system, 

 23 radial trunk of superficial nervous system, 24 radial epineural canal, 25 radial peri- 

 haemal canal, 27, 29 radial canal of water-vascular system, 28 longitudinal muscles, 30 

 radial piece of calcareous ring, 31 retractor muscle, 32 dorsal intestinal vessel of vascular 

 system. 



radial vessels which travel oralwards to the anterior end of the 

 body and then aboralwards in the radii of the body wall, just 



