V.] THE FRESH-WATER MUSSEL. 321 



into it and lay bare its entire lumen its course 

 is as follows. It first passes to the left side down- 

 wards and backwards, and, on nearing the hinder 

 end of the body, turns upwards towards the peri- 

 cardium; it then bends sharply upon itself and 

 passes downwards and backwards, in a line with 

 the free posterior edge of the body-wall. On 

 reaching the middle ventral region it is again bent 

 sharply upon itself, passing upwards and back- 

 wards altogether to the right side; at about the 

 middle of the reproductive gland it becomes 

 suddenly greatly enlarged, and passes (parallel 

 with the first segment) upwards and forwards 

 towards the anterior end of the pericardium. It 

 enters this at its base, and, running upwards and 

 backwards, leaves it postero-dorsally, finally pass- 

 ing over the posterior adductor muscle. (Cf. 

 Sect. B. d. 8.) 



d. Having laid bare the interior of the intestine, re- 

 duce its walls to the condition of median longi- 

 tudinal section and examine the intestinal valve or 

 typhlosole; a thick rich yellow ingrowth of its lining 

 membrane. It is very conspicuous in the first and 

 the enlarged terminal segments. 



e. The labial-palps^ in relation to the mouth. Obtain an 

 undissected specimen, and, after removal from the 

 shell, cut away the anterior end of the body, to 

 the level of the labial-palps ; pin down and examine 

 from beneath. The aperture of the mouth is oval. 

 The labial-palps diminish in size as the mouth is 

 reached and they embrace it in a lip-like fashion; 

 they are confluent on either side and their free 



M. 21 



