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 retracted eye, with its retractor muscle ; 

 the cut attachment of the generative organ ; the zient ; the heart ; 

 pericardium ; aorta ; vena cava ; lung, with pleural membrane ; 

 and the retractor inuscles of the head and inferior antennae. 



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, pericardium, and lung, and is attached to the 

 skin by its border. 



Shell. — Molluscs 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, Arion ater, it consists 

 of a few granules. The shell of Arion hortensis is a little more 

 perfect, the granules being adherent, and measures from l-50th to 

 l-32nd of an inch in its longest diameter. The shell is over the 

 heart and forms a protective covering to that organ. 



Cirailation. — The heart occupies a position in the posterior 

 part of the lung substance, immediately under the rudimentary 

 shell; it is about l-12th of au 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 



