in the larv^ of anisopteeid dragonflies. 131 



Terminology. 



Various names have been applied by the authors cited above to the rectal 

 breathing-apparatus and its parts. As there seems to be little unanimity 

 between them, and as definite English equivalents seem still to be lacking for 

 the names given by the French and German authors to whom the progress 

 of our knowledge is almost wholly due, I offer here the following terminology, 

 which will be used throughout this paper : — 



The Rectum proper consists of all that portion of the hind-gut lying 

 posterior to the dilated ampulla (^pre-o'ectal ampulla) into the anterior end of 

 which the small intestine enters. It consists of two distinct portions, a small 

 posterior or anal portion containing six longitudinal raised epithelial pads 

 (the so-called " rectal glands "), and a much larger anterior or respiratory 

 portion containing the breathing-organs. To this latter portion, with which 

 this paper has to deal, the term Branchial Basket or Gill-basket will be 

 applied. 



The folds of the rectal wall, which project into the cavity of the rectum 

 and perform the respiratory function, will be spoken of generally as the gills 

 or gill-folds. In a gill-fold, each of the two walls forming the eversion will 

 be called a gill-wall. The two gill-walls forming any given fold are there- 

 fore continuous with one another at the free distal border or edge of the 

 gill-fold. Each gill-wall is formed of a thin epithelial matria; with an outer 

 secreted cuticle of excessive fineness. 



The thickened epithelium which occurs at the bases of the gill-folds will be 

 spoken of as the hasal p)ad (" bourrelet epithelial" of Sadones). 



The peculiar mass of tissue which lies at the base of each gill, carrvino- the 

 efferent tracheae, and covered either on one or both sides by the basal pad, 

 will be called the hi/pobranchial tissue (" tissu adipeux^^ of Sadones). 



When the gill-basket of an Auisopterous larva is opened, the gills are seen 

 to lie in six longitudinal rows at equal intervals apart. Each of these rows 

 will be termed a holohranch. Two main types of gill- basket are readily 

 recognizable. In the simpler type, each holohranch consists of a single 

 longitudinal gill-fold, supported at regular intervals, to right and left alter- 

 nately, by smaller cross-folds. This type of structure will be called a Simplex 

 System. In the second and more complicated type, there are no continuous 

 longitudinal gill-folds ; but each holohranch consists of a double row of 

 smaller gills arranged more or less transversely to the long axis of the gill- 

 basket. Each of these rows will be called a hemihranch. The number of 

 gills in a hemibranch varies from twelve to thirty. This second type of 

 gill-structure will be called a Duplex System. 



The exact positions of the gill-rows or gill-folds in a transverse section of 

 the rectum are of the greatest importance, if one would correctly understand 

 the homologies of these organs. Hence I propose to use what I may call the 



