230 PHYLUM ECHINODERMATA. 



its tip is coloured with pigment ; there is no evidence that it 

 has a visual function. In the Diadematidae the skin is provided 

 with numerous shining spots which have the structure of com- 

 pound eyes (vide Sarasin loc. cit.). The sphaeridia have been 

 supposed to have an orientating function. 



The alimentary canal winds through the body from the mouth 

 to the anus (Fig. 170). It is suspended to the body wall 

 by a perforated mesentery, and sometimes the coils are con- 

 nected by a mesentery. Pharynx, stomach, intestine and rectum 

 may be distinguished but are little marked off from one another. 

 The junction of the oesophagus and intestine is often marked 

 by a swelling and in Spatangus by a caecum. There is an 

 assessory intestine or siphon (absent in Cidaroida) which is 



FIG. 170. Sea-urchin divided equatorially (after Tiedemann, from Claus). D alimentary 

 canal, fixed to the shell by the mesentery ; G gonads ; J interambulacral plates. 



given off from it near its commencement (oesophageal region), 

 accompanies it along its inner (axial) wall, and joins it again 

 lower down. It is supposed to allow of the passage of re- 

 spiratory currents of water. In a few genera (Schizaster, Brissus, 

 etc.) there is a second siphon. The walls of the alimentary canal 

 contain muscular elements and, in the first part of the intestine 

 at least, blood lacunae. In the Spatangoida the alimentary 

 canal is always found distended with sand. 



The coleom presents essentially the same parts as in Asteroids. 

 It consists of water- vascular, perihaemal, and perivisceral por- 

 tions, and of the axial sinus. The perivisceral portion occupies 

 the greater part of the interior of the body and is in relation 

 with the coils of the alimentary canal. 



