78 bulletin: museum of comparative zoology. 



Larva II, 2Jf mm. 



The condition of tho body cavity in this larva is essentially as in 

 Larva I, with the exception that the "shelf" has become more pro- 

 nounced, now separating the glomerular cavity (Fig. 32, Plate 3, cav. 

 (jlni.^ from the sub-glomerular cavity {cao. sl/ghn.) as far back as half- 

 way between the two nephrostomes, instead of for a distance of only 

 two or three sections. 



In Larva I the epithelium of the nephrostomes was thickened.-^ In 

 Larva II the thickening of the lower lip of the anterior nephrostome is 

 continued as a band ventrad over the face of the pronephros on to the 

 shelf, where it turns caudad. This is sliown in Figure 32 (tae. e^fh. a), 

 which is from a section passing immediately posterior to the first nephro- 

 stome, whose cilia (omitted by the lithographer) occur in the dorso-lateral 

 angle of the glomerular cavity. When this band reaches the posterior 

 limit of the shelf it passes below it for a very short distance on the lateral 

 wall of the body cavity, turning slightly forward (Fig. 33, tae.e'fh.y). 

 It is as if the band had at first grown directly ventrad from the first 

 nephrostome and had then been pushed backward by the caudad growth 

 of the shelf (compare Fig. /, p. 88). 



Larva III, 21.5 mm. 



There is in this larva a slight but important advance over Larva II. 

 The thickening of the ventral lip of the first nephrostome is distinctly 

 accentuated to form a small thick disk (Fig. 36, evg. 1). From this the 

 band runs ventrad, inclining somewhat caudad, then caudad until free 

 of the shelf, then cephalad, precisely as in larva II. A slight thickening 

 below the ventral lip of the second nephrostome is observable on one 

 side of the body. 



Larva IV, 44 "?^^^- 



This larva shows the beginning of a process which seems to be pre- 

 paratory to the degeneration of the pronephros ; that is, a compression 

 of the pronephros and glomerular cavity in a dorso-ventral direction 

 (Figs. 38, 39, Plato 4). The glomus is also affected, becoming more 

 and more attenuated. In Larva III it was confined to the space 

 between the two nephrostomes, while in this larva it extends anterior 

 to the first and posterior to the second. With the compression of the 



1 The term " thickening " is somewhat of a misnomer. The most conspicuous 

 feature is often not so much a thickening of the epithelium as a crowding together 

 of the nuclei, wliich stain more darkly than tliose of the surrounding tissue. 



