220 K. Ogiinui: 



and owing to the iiew formation of the epithelial tissue after the last 

 ccdysis (flg. 12 ne), all of the rectal gills are completely eliminated from 

 the inner surface of the aiimentary canal. Subsequently the gill lamellae 

 fiise together into compact cell masses and the boundaries of each lamella 

 are entisely lost. There are still observable the sets of fat cells, thongh 

 they are decreased both in size and nnmber. The nuiscles, a considerable 

 development of which was already observable in the preceding stage, in 

 the Space of the basal cushion, becoine more and more conspicouus in 

 this stage, and those situated in the interniediate parts of every double 

 gill rows also grow prominent. Pigment granules are gathered at the 

 tips of these degenerating gills. 



VI. Rectal Tracheal Gills in Mature Imago. 



The bind indestine reaches in the mature imago, its extreme 

 Prolongation in accordance with the complete Stretch of the abdomcn, 

 and the ileum extends straight instead of being more or less covuluted 

 as was Seen in the precedings stages. The whole lengfh of it surpasses 

 three times the length of the rectum. The part where the ileum joiiis 

 with the rectum is strongly constricted, and the latter becomes again 

 dilated. Slightly posterior to this constricted portion and on the external 

 surface of the rectum are six small brownish patches (Hg. 13 RQ), of 

 which three, together with three raised streaks between them, are to be 

 seen only on the dorsal side. These patches are the last remnants of 

 the rectal gills, and the streaks are muscles in their highest development. 

 Examining sections throuhgt this region (fig. 14) we find that the inner 

 surface of the part with the brown patches is lined with thin epithelial 

 cells, under which many pigment granules are collected and is surrounded 

 by thick muscle layer. No trace of the branchial epithelium is left, all the 

 epithelial cells disappear after the extreme degeneration, and it is due to 

 the fate of the respiratory organ out of use in the last stage of life. 



Development of six muscles between two double gill rows is striking 

 but two of them in the ventral position are scarcely distinct from 

 each other, as they are closely connected with the nuiscles in the peri- 

 phery of the degenerated gills. 



VII. liistological Study on the Elimination of Rectal 



Gills and the Regeneration of the Epithelium 



and Fat Cells. 



Prolongation of the Epithelial Tissues of Both the Ends of the Branchial Part. 



hl the foregoing pages I have described : how useless gills are 

 eliminated and how they degenerate at last. Now 1 will briefly deal with 

 the histological changcs of them. After the last ecdysis, each of the 

 double gill rows assumes, by approach of both ends of branchial part 



