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- 

 lamellas ; 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 will be seen running along the top 

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

 d. /3.), the afferent-branchial sinus a well-defined longitu- 

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



