36 CAUDAL GILLS OF ZYGOPTERID LARVvE, 



terid larvae, since their loss causes little or no inconvenience 

 to the growing larva. This has led to the beginnings of 

 the study of the rectum of Zygopterid larvse, as a possible 

 respiratory organ. On this question, Ris(29) appears to have 

 favoured a negative attitude, though Calvert (6) has quite recently 

 made observations on the larvjB of Calopteryx and Hotan'vnay 

 wliich support the original views of Dufour and Hagen. I may 

 add that unpul3lished experiments of my own on the larva? of 

 Diphlehia and Ausfrolpsfes agree closely with Calvert's results. 

 This question must, however, remain over to be dealt with on a 

 future occasion, as much more work needs to be done before we 

 can generalise with any prospect of finality. 



If we turn to the more immediate problem of this papei\ the 

 morphologi/ of the caudal gills, we find verj' little work published 

 on it. Leaving out of account the numerous descriptions of the 

 external form of the gill, in various genera and species (a large 

 number of which have been described, chiefly from Europe and 

 North America), T am only able to indicate one exhaustive 

 study of the morphology of a caudal gill, viz., that by Ris(28) on 

 the large bladder-like gills of Pseudophcea. This is the form of 

 gin which will be dealt with in this paper under the name of 

 saccus or saccoid gill. As Ris' account is by far the most im- 

 portant piece of work so far published on these organs, a full 

 comparison between his results and my own will be given in 

 the section devoted to Saccoid Gills. Quite recently, Calvert (6, 

 p. 391) has sectioned the peculiar gills of ThauTnatoneuy^a, and 

 given a short but excellent account of their internal structure, 

 agreeing in many points with that given by Ris for Pseudophcea. 

 I have not been able to find any detailed account of the mor- 

 phology of the triquetral gills of the subfamily Calopteryginw., 

 nor of the commoner lamellar gills of the Lest idee and most 

 Agrionidce. Nor is there, as far as I know% any published work, 

 in which a comparative study of the various forms of caudal 

 gills known to exist has been undertaken, with a view to indi- 

 cating the phylogenetic course of development of these organs. 

 The only paper that can claim to offer any ontogenetic results is 

 that by Bal£our-Browne(l). This paper, how^ever, does not go 



