492 MAYOR. 



At the oral end the ectoderm of the body wall is folded in to form 

 the oesophagus and the filaments of the first two pairs of mesenteries. 

 At its oral end the oesophagus forms a complete tube. Aborally it 

 becomes divided dorsally so that it extends for some distance as a 

 scoop-shaped structure, U-shaped in cross section. In the following 

 descriptions the point where the oesophagus ceases to be a complete 

 tube and becomes scoop-shaped is expressed by saying that it is 

 interrupted dorsally. 



b. Description of the Mesenteries, Mesenterial Filaments and Gastro- 

 vascular Cavities of the Larvae Studied. 



Group I. Larva A. 



In this larva (text, Fig. 1) the first two pairs of primary mesen- 

 teries are well developed and there are indications of the third and 

 fourth pairs. The larva was fixed in Flemming's chromo-aceto- 

 osmic mixture and the sections were stained with Heidenhain's iron 

 haematoxylin. The aboral end of the larva was sectioned first 

 and the sections are almost exactly transverse, the ventral side of the 

 oral end, however, extends six sections beyond the dorsal side, a 

 deviation from the transverse which cannot be detected at the aboral 

 end. The sections are 7 micra thick and there are 56 of them in all. 

 Text-figure 1 shows a.t A, a reconstructed longitudinal section passing 

 through the mesenterial filaments of the first pair of mesenteries. 

 The horizontal dotted lines are drawn in the planes of sections and 

 each of the numbers at the side of the figure indicates the number 

 of the section to which the line corresponds, beginning with the oral 

 end. The figure shows that the aboral end of the larva is flattened 

 and contains a slight concavity. This shape may be due to con- 

 traction on the application of the fixing fluid, but larvae of this form 

 were observed in the living state. At B, C, D, E, and F, in Figure 1, 

 are shown the 14th, 18th, 23rd, 30th, and 41st sections respectively 

 from the oral end. Photomicrographs of sections 15, 24, 30, and 46 

 are shown in Plates 1 and 2. 



The oesophagus is irregularly circular in cross section (Fig. I, B) 

 and extends aborally in the axis of the larva. The fact that it appears 

 nearer to the dorsal side in the figure Is due to the section having been 

 cut somewhat obliquely. The larva has been cut so that the oesoph- 

 agus appears as a complete tube first in section 6, the ventral side 



