176 PHYLUM ECHINODEBMATA. 



and subjacent connective tissue. In a typical case* each eye consists of a 

 number (50 to more than 100) of funnel-shaped ectodermal pits, the cells of 

 which contain the red pigment and end in a clear highly-refractile rod 

 which projects into the cavity of. the pit. The pit is closed towards the 

 exterior by the cuticle, which may have on its inner side a lens-like 

 thickening. 'The pigment cells are continued internally as fine fibres which 

 join the nervous tissue of the radial nerve. The pits increase in number 

 with the growth of the animal and they appear to contain a transparent 

 gelatinous tissue. In some cases there are no pits and the pigment cells 

 are distributed uniformly over the surface of the ocular cushion (Astro- 

 pecten pentacanthus, mulleri). 



The alimentary canal begins with a mouth which is placed 

 in the centre of the actinal surface of the disc in the buccal 

 membrane, and leads into a short oesophagus or directly into 

 the spacious stomach (Figs. 127, 132). The oesophagus passes 

 quite gradually into the stomach from the abactinal part of 



FIG. 127. Longitudinal section through the disc and an arm of Solaster endeca (from Claus, 

 modified after G. O. Sars). mouth leading directly into the wide stomach ; 4 anus ; 

 L hepatic diverticulum of the stomach ; G gonad ; Md madreporite ; Js rectal caecum ; 

 Af tube-foot. 



which two caecal diverticula (hepatic) are given off into each 

 arm. From the stomach a short rectum, which receives the 

 rectal caeca, leads to the anus, which opens on the abactinal 

 surface in interradius /. // (Figs. 83, 122). 



The buccal membrane is the part of the body-wall round the mouth 

 in the oral depression and is devoid of calcareous structures ; the circular 

 muscular fibres in it act as a constrictor .and the longitudinal as dilators 

 of the mouth opening. The oesophagus passes without any line of de- 

 marcation into the stomach ; it is beset with ten glandular diverticula in 

 Echinaster and Cribrella. The stomach is a spacious, thin-walled sac, 

 divided by a horizontal fold (absent in the Astropectinidae without an 

 anus) into an oral (cardiac) and aboral portion (pyloric sac). Its walls are 

 often considerably folded, so that it appears lobed. From its aboral 

 portion the stomach gives off the hepatic or pyloric caeca (Fig. 128), one 

 pair into each arm. These are tubular structures beset with numerous 



* Pfeffer, ZooL Jahrb. Anat., 14, 1901, p. 523. 



