500 MAYOR. 



in that section. From sections 57 to 61 the size remains nearly 

 constant, but below this it decreases until they end, on the right at 

 section 106 and on the left at section 102. 



The extent and position of the mesenteries is shown in the accom- 

 panying table. 



Only mesenteries / to IV are complete. 



The gastrovascular cavity extends orally between the mesenteries 

 as in larva B. At the oral end however it is found between the first 

 four pairs of mesenteries only (Fig. ^, A). Where these diverticula 

 of the gastrovascular cavity occur the fifth and sixth mesenteries do 

 not reach to the oesophageal wall. 



Group III. Larva H. 



This larva, which is in about the same stage as larvae F and G, 

 is very much contracted along the oral-aboral axis, the aboral surface 

 being retracted to form a deep concavity. The individual was fixed 

 in a saturated solution of corrosive sublimate plus 1% acetic acid. 

 The sections were stained with Heidenhain's iron haematoxylin and 

 Congo red, and the oral end of the larva was cut first, the right side 

 being slightly in advance. The 71 sections are each 7 ju thick. 



The oesophagus is elliptical in cross section, the long axis of the 

 ellipse being dorso-ventral. It is interrupted on the dorsal side in 

 section 14 and on the ventral side in section 21, its ventral side extend- 

 ing therefore 7 sections beyond its dorsal side. 



The mesenterial filaments of the first and second pairs of mesen- 

 teries, the only ones present, extend nearly the whole length of the 

 larva, almost but not quite touching the mesoglea of the aboral end; 

 their position is as follows: those of the first pair of mesenteries on 

 the right reach from section 22 to section 41, and on the left from 

 section 23 to section 44; those of the second pair of mesenteries on 

 the right reach from section 22 to 41 and on the left from section 23 



