218 K Oq-unia: 



The epithelial tissues arranged in a unicellular layer are variable 

 in their size but usually not more than several times the length of a cell, 

 and these cells or epithelial tissues are froni the epithelial cushions of 

 the gills. Such cells could be found nowhere but in the cushion, 

 and a close study shows that these two are identical. On one inargin 

 of this detached epitheliuni, or the tissue which was once the epithelial 

 cushion, is recognizable a thin cuticular covering passing into other parts 

 of cuticula of gill surface, white its opposite margin shows occasionally 

 irregulär appearance whithout any coverings. Aniong the foldings of gill 

 exuvlae, furthermore, there are found many like tissues in degeneration 

 or decay. This fact suggests that these tissues would, sooner or later, 

 decay leaving behind only the cuticular substance in the nyiuphal skin. 



The Remaining Rectal Gills. 



The rectum modified into the rectal gills, is eliminated behind the 

 ileum by the considerable growth of the postbranchial rectum. It shows 

 as a whole, a semilunar shape on sagittal sections of the bind 

 intestine ; the gill lamellae remaining within are arranged somewhat radially 

 (fig. 9 Ol), directing their tips to the centre of this part of the rectum, 

 and the fat cells in the basal cushion are still visible in the same part. 

 Of course the gill lamellae, in this period, has no longer cuticular covering, 

 and the hypodermal cells of it are fused together including many tracheal 

 branches. At the part of the basal cushion, where the epithelial cushion is 

 situated, epithelial cells entirely disappear, leaving the basement membrane 

 only, white at the opposite wall many small cells are yet to be seen lying 

 upon the same membrane (fig. 10 ep). From these data we are able 

 to conclude that the cushion cells are cast off from the gills, 

 together with the accompaning cuticular coat and leaving the 

 basement membrane in the adult body. The irregularity of one 

 margin of detached cushion above noted is due to this process 

 of gill transformatton during the last ecdysis of nymphal life (fig. 10 C). 

 The basement membrane remaining usually shows many convolutions 

 near the gill lamella (fig. 10 bm), denoting the contraction which took 

 place at the same time whith the elimination of the epithelial cushion. 



Periodical falling and regeneration of the epitheliimi has becn 

 studied at first by Bizzozero (1893), later by Rengel (18981 and more 

 recently by Deegener (1902i, in the mid-intestine of several water 

 beetles\), and the mode by which the epitheliuni falls off, leaving the 

 basement membrane behind it, shows very close resembrance to the case 

 of epithelial cushion in Odonata (compare with Rengel's figures-) 2 und 3). 



\) Such as Hydrophilus, Hydrobius and Hydrous. 



-) Figure 1 and 3 were cited by Deegener (Handb. Hut. p. 281, 282 1. 



