ON THE LIFE-HISTOBT OF PEDTOELLINA. 149 



Fig. 3 represents a horizontal section of a larva not long 

 after its fixation : the occurrence of brain (= " dorsal organ," 

 V. No. 4), oesophagus, and rectum in the figure sufficiently de- 

 fines the level of the section. The epistome is cut in the region 

 of its greatest thickness, whilst at the summit of the anal 

 cone is seen the depression into which opens the anus. By- 

 comparing this with fig. 1, it will be observed that the anus 

 has already altered its position, since it is now directed some- 

 what forwards, the rectum being more nearly parallel to the 

 stomach than before. The cells of the vestibular epithelium 

 are very high at the sides of the anal cone, and are character- 

 ized by the special readiness with which they take up colouring 

 matters. 



Fig. 5 represents a horizontal section through the apices of 

 the epistome and anal cone of another individual of the same 

 age. The epistome is here seen to be continuous, at each side, 

 with a fold of vestibular epithelium ; epistome and folds 

 together forming (as seen in this section) a horseshoe-shaped 

 ridge partially embracing the sides of the anal cone, in which 

 region the two lateral folds become evanescent. The result of 

 this arrangement is the formation of a somewhat deep ciliated 

 groove (o. g.) running round the greater part of the vestibule, 

 and passing in front into the transversely elongated, funnel- 

 shaped mouth. Posteriorly, however, owing to the disappear- 

 ance of the lateral folds, the oral grooves fade away at the 

 sides of the anus, where vestibule and oral grooves conse- 

 quently appear continuous in such a section as that represented 

 in fig. 5. The relations of these structures will become more 

 clear on reference to fig. 4, a larva somewhat older than those 

 previously described, the section passing transversely through 

 the region of the anal cone, in the plane a b in fig. 1. At the 

 sides of the anal cone are the two lateral portions of the vesti- 

 bule (/. v.), these structures being separated from the oral 

 grooves by the folds already mentioned. In the more anterior 

 sections of the series, the lateral folds become continuous with 

 the epistome, and the oral grooves with the mouth. Further 

 back, on the contrary, the folds become lower, and finally dis- 



