146 Report of the Microscopical Section. [Sess. 



pair of compound eyes ; there is also on each side of the head 

 a small transparent spot called a fenestra; by some these 

 spots are supposed to be the remains of simple eyes. 



The thorax consists of three parts — pro-thorax, meso-thorax, 

 and meta-thorax. 



The abdomen consists altogether of ten segments, of which 

 the first six are quite distinct, the seventh, eighth, and ninth 

 are telescoped into each other, and the tenth is split. The 

 tracheae communicate externally by means of pores called 

 stigmata, of which two pairs are in the thorax and eight pairs 

 in the abdomen. 



When the tergal portion of the exoskeleton is removed the 

 internal organisation can be studied. It consists of the — 



Heart, a long slender tube lying immediately under the 

 exoskeleton and above the alimentary canal. 



Alimentary canal, consisting of the following parts : Salivary 

 glands, which opea into the mouth. Each gland is composed 

 of two leaflike lobes, with a single duct; there is also a 

 salivary bladder on each side. Gullet, slender at first, then 

 dilating into the cro}-), which is large and situated in the 

 abdomen. Gizzard, at the posterior end of the crop, provided 

 with a thick muscular wall, and six large teeth and numerous 

 smaller teeth between the large ones. Stomach, with pyloric 

 cpeca and Malpighian tubules, which probably function as a 

 liver and kidneys. Intestine, small and large, ending in anus 

 opening posteriorly. 



On removing the alimentary canal the nervous system is 

 exposed, as also the reproductive organs. 



Molluscan Anatomy. — The " Common Snail " {Helix aspersa) 

 was taken as the type. The Mollusca may be defined as 

 " animals with a soft body, without segments, naked or 

 covered with a shell of one or two valves composed of 

 carbonate of lime secreted by a fold of the skin — the mantle." 

 In the body there are three great divisions — shell, foot, and 

 head. The Mollusca are generally sluggish, although some 

 bivalves are spasmodically active. This sluggishness is 

 favourable for the secretion of lime, of which the mollusc 

 has need for the building up of its shell. 



The foot in the snail is largely developed. It is, as its 

 name indicates, the organ of locomotion. 



