28 Invertebrate Zoology. 



is laterally compressed often to such an extent that its walls 

 are actually in contact. A double plate is thus formed 

 which lies in the median plane (perradially). The flattened 

 oesophagus opens near the aboral pole into a much smaller 

 stomach. 



The thick layer of jelly-like tissue that lies between the 

 walls of the oesophagus and stomach and the external integ- 

 ument is traversed by certain gastro-canals, which, as diver- 

 ticula, radiate from the stomach. 



If a living animal has coloring matter forcibly injected 

 into the opening of the mouth, the course of these canals 

 will be beautifully demonstrated. 



There are eight large gastro-canals that take their origin 

 from the stomach, of which four arise laterally and in the 

 second perradial plane and four arise interradially. A single 

 pair of very small canals extend aborally in the first per- 

 radial plane from the stomach to the region of the " sensory 

 body." 



Of the four lateral canals, one pair, the more external 

 (tentacular vessels), extend directly to the tentacles, while 

 the other, deeper pair, run along the wall of the oesophagus 

 as the Ksophageal or gastric vessels. Each of the four inter- 

 radial canals branches dichotomously soon after leaving the 

 stomach. Eight meridional vessels are thus formed which 

 lie under and parallel to each ctenophoral row. 



It is on the walls of the meridional vessels that the eggs 

 and spermatozoa are developed. These reach the sea-water 

 after passing through the stomach and oesophagus. 



