ek ee oe 
TRANSACTIONS. 183 
generative orifice. There is a caudal slime gland, with a very 
short duct. The gland is in the substance of the skin. 
After killing the slug, with the scissors cut down the centre 
of the foot, commencing behind the buccal mass, then pin down to 
the dissecting table by the edges of skin ; remove the visceral mass, 
and lay aside in water for future examination. There will remain 
attached to the skin the refracted eye, with its retractor muscle ; 
the cut attachment of the generative organ ; the vent; the heart ; 
pericardium ; aorta; vena cava; lung, with pleural membrane ; 
and the retractor muscles of the head and inferior antenne. 
Respiratory System.—Breathing is carried on through the 
pulmonary aperture which leads into the lung cavity. In inspira- 
tion, the muscle which lines the floor of the mantle contracts and 
bulges it up, and air is drawn in when the muscle relaxes ; the 
mantle flattens and the air is expelled. The pleural membrane 
envelops the heart, pericardinm, and lung, and is attached to the 
skin by its border. 
Shell—Molluses without any external shell are called slugs ; 
those with external shells are called snails. In slugs, between the 
muscular floor of the mantle and the outer skin, there is a shell 
more or less developed. In the black slug, Avon ater, it consists 
of a few granules. The shell of Avion hortensis is a little more 
perfect, the granules being adherent, and measures from 1-50th to 
1-32nd of an inch in its longest diameter. The shell is over the 
heart and forms a protective covering to that organ. 
Circulation.—The heart occupies a position in the posterior 
part of the lung substance, immediately under the rudimentary 
shell; it is about 1-12th of an inch in its longest diameter ; is 
enclosed in the pericardium ; the whole, as well as the lung, is 
covered by the pleura. The heart is a muscular sac divided into 
two cavities—an auricle and ventricle. It has a rythmical action, 
beating about 40 times in a minute, and may sometimes be seen 
pulsating externally a little to the left of the centre of the mantle. 
The heart of a frog or fish if removed at once after death from the 
body will continue to beat for some time. This power of rythmic 
contraction is sustained by small nerve centres in the substance of 
the heart, which are called ganglia. If those ganglia be destroyed, 
rythmic movements cease. I do not know whether the heart of a 
slug will continue to beat for any time after removal from its 
natural surroundings ; but I have seen the heart of a slug beat 
for an hour after the animal had been cut up and all the viscera 
