350 



G. G. SCOTT. 



and second the tendency of the walls to resume their somewhat 

 cylindrical form. Extending the length of the surface of the 

 rectum are six longitudinal bands or muscles (Fig. 4, RM} a 

 dorsal pair, a lateral pair and a ventral pair. These muscles are 

 equally distant from each other. To each of them internally is 

 attached two rows of leaflike structures, the rectal gills (Fig. 4, 

 R G) which extend into the cavity of the rectum. The dorsal 

 trachea (Fig. 4, D] sends branches to the dorsal and lateral rectal 

 From the ventral trachea (Fig. 4, V] branches pass to 



gills. 



D- 



--R M 



V R G 



FIG. 4. 



FIG. 5. 



the ventral rectal gills. Fig. 5 shows the relation of the 

 gills (Fig. 5, R G) to the longitudinal muscles (Fig. 5, L M}. 

 In this figure the gills are pressed out laterally. The shortest 

 edge of each gill is attached diagonally to the muscle so that the 

 lines of attachment of the two rows of gills of the same muscle 



have the appearance of a succession of 

 "V 's," the diverging ends being toward the 

 posterior. Each gill is somewhat triangular 

 in shape (Fig. 6, R G) the proximal edge 

 attached to the longitudinal rectal muscles 

 being much shorter. At one corner of this 

 shorter side is a somewhat triangular space 

 (Fig. 6, TR SP} differentiated from the rest 

 in general appearance. Sadones '95 de- 

 scribes it in the gills of other species and 

 gives no decisive clue as to the function of it. He ascribes, how- 

 ever, an excretory function to it. 



On one surface of the gill are three small pads (Fig. 6, PD} 

 by which it is separated from the adjacent gills. The pads allow 

 a thin film of water to form between each gill. Each gill is about 



-RG 



FIG. 6. 



