128 MR. J. M. BROWN OX THE ANATOMY OF 



In the walls of the pits the following layers can be distinguished (PL 25. fig. 10) : — 



1. The hypodermis, consisting of regular block-shaped cells with large nuclei (hyp.). 



2. The laminated cuticle (cut.), moderately thick, having the same characters as the 

 external cuticle of the body-wall. Prom this cuticle there arise large numbers of 

 chitinous hair-like outgroicths (ch.), projecting into the stigmatic chamber and forming a 

 very dense lining to it. Each hair gives rise to side branches which unite with those of 

 neighbouring hairs, in much the same way as was noticed in the hairs of the Y-pieces of 

 the spiracle cover. This lining covers the whole internal surface of the chamber, except 

 where the bunches of tracheae arise, and seems to take the place of the tsenidia 

 common to tracheae. [In one of my series of sections these same hair-like outgrowths 

 occurred in a branch tracheal tube in the mid-region of the body, in place of the usual 

 spiral thread.] 



At frequent intervals along the length of the stigmatic chambers bunches of clear- 

 walled tubes, without " sj>iral thread," and enclosed in a nucleated sheath, take origin 

 (Pis. 24 & 25. figs. 6 & 10, tr.hd.). These bundles radiate on all sides from the chamber, 

 passing outwards and somewhat forwards, divide into smaller and smaller bundles by 

 the separation of groups of tubes. A short distance from the stigmatic cavity the 

 nucleated sheath ceases, after which large nuclei occur at rather rare intervals amongst 

 the tubes, and most frequently at points where the groups of tubes separate from the 

 main bundle (PI. 25. figs. 10 & 12). Nearing the body-wall of the posterior segment the 

 groups become separated entirely into individual tubes (without sheath), which in their 

 turn branch until, becoming excessively fine threads, they become attached to the 

 inner surface of the body-wall, where they form an apparently web-like covering. 

 Entangled amongst these tine tubules, corpuscles of the body-cavity fluid ("blood") 

 occur in large numbers (PI. 25. figs. 13 & 14, corp.). 



The whole of this region apjiears to be an exceedingly complex respiratory structure. 

 The blood on its way to the dorsal vessel in this region, which it enters through a 

 posterior orifice— through which the strongest inflow seems to occur, — must necessarily 

 circulate amongst these fine air-tubes, and probably here occurs some of the gaseous 

 exchange. 



The two stigmatic chambers are connected together by a loop tracheal tube with 

 tamidia, which branches from the inner side of each pit about halfway forward, 

 and curving ventralwards, meets its fellow in the middle line. Prom this loop-tube two 

 branches arise, pass ventralwards and forwards to supply the hindmost pair of blood- 

 gills. The anterior and smaller pairs of blood-gills receive their tracheal supply from a 

 ventral branch arising at the anterior end of each stigmatic pit. Hence the whole 

 tracheal supply to this respiratory region arises either directly or indirectly from the 

 stigmatic chambers, where one may suppose the air to be most completely renewed. 



The ventral processes or " blood-gills " (Pis. 24 & 26. figs. 2, 3, 4, & 18) are hollow tube- 

 like outgrowths of the body-wall. They are divided longitudinally into two channels by 

 a fine membrane-like partition supported by the large air-tube which traverses the 

 structure. The blood flowing backwards along the ventral sinus of the body, circulates 



