4-' 



'S 



RESPIRATION OF MESOGOMFH[-S- 



hack, the lateral tracheae, likewise a normal structure and here 

 in no wise enlarged, join the dorsals. VV are the ventral trunks, 

 and LI the laterals. 



At this point there is g-iven off a special trach;e, the jxistdor- 

 sal, on each side. This passes back to the caudahnost part of the 

 digestive tract raid to the caudal appendages. The postdorsals 

 are shown as PD in Figure 2. 



The distribution of trachese is similar as regards the 

 branches from the ventral trunks. There is a ]>()stventral also 

 on each side, passing to the caudal end of the rectum. See, 

 figure 3. 



The enlarged hollow organ, R, is the rectum or gill chamber. 

 It lies in the seventh to ninth somites of the abdomen. The la^t 

 abdominal somite is occupied hy a plain canal, the anal canal, 

 AC, which is not respirat(jry. 



V 'I V 



Fig. 3. — Ventral View. 



It must be noted that the dorsal trunks are larger ilian the 

 ventrals, and have many more branches. In both trunks there 

 arise branaches on both sides of the trunk. The larger size and 

 greater branching of the dorsal trunks is explained b\' the fact 

 that branches from them pass io two-thirds of the area of the 

 rectum, while only the ventral third is connected t" tlie ventral 

 trunks. 



Figure 4, a cross-section of the rectum and trunks, shoA\ •- 

 this distribution plainly. 



The outside of the rectum has upon it six dark longitudinal 

 lines, along which the tracheal branches enter it. These lines 

 are the bases of the gill-folds within the rectum, and their neck- 

 lace-like character is caused by the occurrence of bases for 

 definite gills within the rectum. Between the rows of gill-bases 



