II ABDOMINAL VISCERA 23 



The small intestine becomes continuous, posteriorly, with 

 a much wider tube (Figs. 3 and 7, rci), lying against the 

 dorsal wall of the abdomen, and called the large intestine or 

 rectum. It is continued into a short tube, the cloaca (ct), 

 which passes backwards, between the backbone above and 

 the pubis below, to open externally by the vent. Thus the 

 mouth-cavity, pharynx, gullet, stomach, small intestine, rec- 

 tum, and cloaca form a continuous tube, opening externally 

 at each end, by mouth and anus respectively, and, for the 

 greater part of its extent, contained within the body-cavity. 

 The whole tube is known as the enteric or alimentary canal. 



Attached to the mesentery, close to the anterior end of the 

 rectum, is a rounded body of a deep-red colour, the spleen 

 (Figs. 3 and 7, spl). Quite at the posterior end of the 

 abdominal cavity a very thin-walled and very transparent 

 sac (ti.bl) will be seen, connected with the ventral surface of 

 the cloaca, and varying very much in size according to its 

 state of distension. This is the urinary bladder , which 

 communicates by an aperture (Fig. 7, bl') with the cloaca, 

 and when distended will be seen to be a bilobed sac of 

 considerable size. 



If your specimen should be an adult female, and the 

 time of year approaching the breeding season, you will 

 already have observed, as the most prominent organs in the 

 body, two large, lobed structures of a dark colour, protrud- 

 ing one on each side, and partly obscuring the view of the 

 other organs. Each (Fig. 4, /. ovy') contains an immen.se 

 number of small globular bodies, half black and half white, 

 and is suspended to the roof of the body-cavity by a sheet 

 of peritoneum. These bodies are the ovaries, or organs for 

 the manufacture of the eggs ; the rounded bodies of which 

 they are largely composed are the eggs themselves. To 

 each ovary is attached a curious structure {cp. ad) of a 



