IV.] THE COMMON SNAIL. 283 



into the respiratory orifice and make an incision along 

 the right-hand border of the sac, cutting clear of 

 the excretory organ. Carry a second incision along 

 the thickened base of the sac towards its left side, and 

 reflect the whole. Note : 



a. The floor of the pulmonary chamber; thin and 

 transparent, there being seen through it the dead- 

 white reproductive organs. 



b. The rectum, a thin walled tube coursing along the 

 floor of the pulmonary chamber at its extreme right 

 hand border. 



Follow it to the anus. It opens, at the base of 

 a groove-like depression in the lip of the respira- 

 tory sac, below and to the right of the respiratory 

 orifice. 



c. The excretory gland, a considerable yellowish mass, 

 lying in the posterior right hand end of the pul- 

 monary sac. Make an incision into it and wash 

 out its contents ; note the thickened spongy texture 

 of its glandular lining, which is thrown into a series 

 of folds by the underlying blood-vessels. 



d. The excretory duct, a yellow thin-walled tube, 

 running to the right of and parallel with the rectum. 

 Its orifice lies a little above and to the right of the 

 anus. Insert a blowpipe into this and inflate the 

 whole ; note that the duct is continued along the 

 right hand border of the gland to its summit. 



If examined minutely, it will be seen that the 



excretory duct is continued on beyond the above- 



. named orifice as a well marked excretory groove, 



whose walls are contractile. This courses over the 



base of the rectum dorsally to the anus, and, passing 



