324 STEPHEN G. RICH 



also to the adjacent gill bases forming the mesal half of the 

 next double row. The lower or more ventral half of this double 

 row and the upper nearer half of the adjacent row on the ven- 

 tral side are supplied by the similar branches from the ventral 

 side of the dorsal trunk and the postdorsal trachea. The post- 

 dorsal here has six branches in some specimens. The tip of the 

 postdorsal passes on to the anal canal. 



The arrangement on the ventral side is different. The ven- 

 tral trunks and the postventral tracheae supplj^ only the mid- 

 ventral double row and the adjacent half of the next row on 

 each side. There are branches on both sides of the ventral as of 

 the dorsal trunk, but each of these divides only to a single row 

 of gill bases (fig. 3). The ventral trunk gives off on each side 

 five branches, each of which divides at once, dividing again just 

 before entering the gill base, exactly as with the branches already 

 described. 



Thus there are two single rows and three double rows supplied 

 from the dorsal tracheae, and one double row and two single 

 rows supplied from those on the ventral side. The cross-section 

 (fig. 3, modified from a MS. figure by Miss Elizabeth Andrews) 

 will perhaps make the relations of tracheal trunks to gill bases 

 clear. 



If we turn to the interior of the rectum, we shall find there 

 six longitudinal folds, each corresponding with the middle of a 

 double row of gill bases. Each row is supported on each side 

 by a series of buttress folds, which lie over the gill bases. Figure 

 4 shows this in plan: the buttress folds pass at about 45° cephalad, 

 and are recurved at their tips, overlapping the similarly re- 

 curved tips of the adjacent row of folds. These folds, main and 

 buttress, are the actual respiratory parts of the gill-chamber. 

 The buttress folds are alternate on each side of the main fold. 



The base of all these folds is filled with a loose mass of fatty 

 tissue, made up of ordinary trophocytes, through which the 

 tracheae pass up into the respiratory organs proper. On the 

 base of each buttress fold are two plates of thick epithelium, 

 one on each side of it. These are oval in shape (fig. 5) and lie 

 across the base of the fold. These, discovered by Chun ('76), 



