Chap. XV.] THE SNAIL. 293 



The snail can, on occasion, completely retract itself into its 

 shell. At such times the collar is exposed so that respiration can 

 be carried on through the pulmonary aperture, which rhythmically 

 opens and shuts. When the snail retreats into its shell for a pro- 

 tracted period, e.g. during the winter months, there is formed across 

 the opening of the shell an epiphragm or hybernaadum, consisting 

 of hardened mucous impregnated with calcareous matter. 



If a snail be completely immersed its tissues absorb a large 

 quantity of water. It may then be rapidly killed by placing in 

 moderately hot water. It should thus die well extended, with 

 its tissues flaccid. After making out the external apertures 

 (except the minute orifice of the renal duct), and inserting a 

 probe into the pedal gland which extends into the foot, and 

 may be opened out now or at a later period the student should 

 remove the shell bit by bit, being careful not to injure the deli- 

 cate tissue of the digestive gland, and should then pin out the 

 snail under water. A probe inserted into the pulmonary aper- 

 ture is seen to pass into the pulmonary chamber, which is roofed 

 over by the mantle. This mantle-roof, in which a number of 

 blood-vessels are seen to ramify, unites along the collar with the 

 dorsal body -walls, except at one point which remains open as the 

 pulmonary aperture. Along the right-hand side of the pulmon- 

 ary chamber runs the rectum. 



Dissection is best begun by cutting along the base of the 

 collar, so as to sever the connection between the mantle and 

 collar and the dorsal body-wall. An incision should then be 

 carried up just to the right of the rectum, so as to leave the 

 rectum attached to the mantle. The roof of the pulmonary 

 cavity may now be examined from its ventral aspect. In it is seen 

 a white tongue-shaped mass, the renal organ (Fig. 85, B., ren.), 

 to the left of which (in this position) is the pericardial cavity, 

 on opening which the auricle (au.) and ventricle (v.) of the heart 

 will be seen. The floor of the pulmonary chamber is seen to 

 be constituted by a white muscular septum, which separates it 

 from the body-cavity beneath. This should now be opened, 

 -and an incision should be carried forward with scissors to the 

 anterior end of the head. A large crop, and anteriorly a buccal 



