v.] THE FRESH-WATER MUSSEL. 327 



2. Make a perforation in the roof of the ventricle, and 

 pass the injecting apparatus through it into one of the 

 auriculo-ventricular apertures; inject under a steady pres- 

 sure, wash and examine the undissected animal under 

 water. There will be seen — 



a. The pallial sinuses ; a series of irregular channels 

 permeating the substance of the pallial lobe. 



b. The efferent pallial vessel; a circular trunk, running 

 along the upper surface of the thickened mantle- 

 edge; it receives branches from the pallium at all 

 points. 



Trace it upwards — it follows the line of attachment 

 of the pallial-lobe to the underlying organs (see 

 Sect. B.); it is coincident, along the line of attach- 

 ment of the gills, with the efferent-branchial sinus 



c. Remove the greater portion of the pallial-lobe, and 

 examine the efferent-branchial vessels — a series of 

 short parallel trunks, lying in the walls of the gill- 

 lamellae ; each is formed by the confluence of 

 lesser vessels, coming in from the gill substance. 

 They open into 



d. the efferent branchial sinus, a spacious chamber 

 overlying the gills ; in the- pericardial region it 

 becomes expanded to form the auricle (Cf. Sect. 

 F. 2. b.). 



3. Remove the rest of the mantle-lobe and open up the 

 underlying external gill-lamina, along its line of junction 

 with the same. There mil be seen running along the top 

 of the gill, at the base of the suspensory ligament (Sect. B. 

 d. j3.), the afferent-branchial sinus — a well-defined longitu- 

 dinal blood-space; make a perforation in this and inject, 



