IN THE LARV^ OF ANISOPTERID DRAGONFLIES. 133 



trachea divides up into two large branches, the secondary efferent trachece. 

 These enter the gill-basket in twelve separate rows. Their methods of 

 branching now vary according to the particular type of gill-basket which 

 they are to supply. Their branches may be spoken of generally as the gill- 

 efferents or simply the gill-t7'achece. 



These gill-trachese branch and re-branch again within the gill-folds until 

 they finally give off a very large number of very minute trachese, which form 

 the last term in this tracheal series. These are the capillaries, capillary loops, 

 or tracheal loops of the gills, by means of which the oxygen is received from 

 the circumambient water in the rectum into the gills themselves. Not one 

 of those tiny capillaries ends blindly; every single one is a complete loop, as 

 Oustalet has clearly shown. 



Terminology of the Various Types of Gill. 



Five principal types of gill will be recognized in this paper. Of these 

 two belong to the Simplex System and three to the Duplex. Subordinate 

 types will also be recognized under one at least of these principal types. 



In the Simplex System, the two principal types are : — 



1. The Undulate Type, in which the free edge of the gill-fold is thrown 

 into complicated undulations or waves. (Text-figure 2.) 



2. The Papillate Type, in which all except the basal portion of the gill- 

 fold is split up into numerous long slender papillae, each of which carries an 

 axial trachea from which the capillaries arise. (Text-figure 3.) 



In the Duplex System, the three principal types are : — 



3. The Implicate Type, corresponding fairly closely to 1 in the Simplex 

 System. In each hemibranch the gills are folded and grooved in a compli- 

 cated manner, and the two series forming each holobranch lie so that their 

 curved distal borders come into close relationship alternately with one another 

 and tend to overlap. (Text-figure 5.) 



4. The Foliate Type, in which the transverse gills forming each hemibranch 

 appear in the form of separate leaves or folise. 



Under this type we may distinguish two very different sub-types : 



4 a. The Normal Foliate Type, in which each folia is undulated alono- its 

 free edge, and bears no papillae. (Text-figures 6-7.) 



4 h. The Papillo-Foliate Type, in which each folia is folded over into 

 a kind of oval " hump/^ bearing numerous tiny papillae. These papillae 

 have no axial trachea, and only carry capillaries. (Contrast Type 2 ) 

 (Text-figure 9.) 



5. The Lamellate Type, in which the gills forming each hemibranch 

 appear as separate flat lamella or plates projecting into the rectum. (Text- 

 figures 11-12.) 



LINN. JOTJRN. ZOOLOGY, VOL. XXXIII. ]() 



