BY R. J. TILLYARD. 



49 



by a comparison of Text-fig. 1 with Plate iii., figs.21, 22. From 

 Text-tig. 1, we see that, at the line of junction of the gill-base 

 with the eleventh segment (which is the line of the breaking- 

 joint), the cuticle becomes suddenly narrowed, and, in particular, 

 the tougher outer portion of the cuticle, which is particularly 

 thick on the basal piece and at the extreme base of the gill, is 



Text-fig. 1. 

 Hasal piece {Bp) and breaking-joint {hj) of right lateral gill of Aryiolestes 

 icttromelas Selys, ( x 87). Drawn from a cleared whole mount. 

 Jftjo, tenth abdominal segment; O, base of gill; Lhy, hypoderm-laj^er 

 across breaking- joint; fr, single main trachea approaching gill; //•', its 

 tAvo divisions within the gill. Rest of lettering as given on p. 109. 



greatly reduced. Thus the gill is not only ^flexible about this 

 joint, but fairly easily detachable also, by reason of the ease 

 with which the thin cuticle can be torn awa}-. 



Now if we examine the breaking-joint in transverse section 

 (Plate iii., figs.21, 22), we shall see that, just at this point, the 

 hypodermis grows out into a delicate transverse layer of 

 elongated columnar or spindle-shaped cells, which almost com- 

 pletely close in the lumen of the organ. The hsemocoele, which, 

 in the basal piece, is broad and undifferentiated, is here reduced 

 to two narrow canals {dh, vh), which are the beginnings of the 

 dorsal and ventral blood-canals of the gill itself. The rest of 

 the hsemocoele is closed up, except for the excessively narrowed 

 channels in which the main tracheae (tr) run. When the gill is 



