THE DOGFISH 13 



and displaying the spiral valve. Sketch this structure. What pur- 

 pose can you suggest for it ? 



Cut through the rectum and through the liver and stomach 

 about an inch from the anterior ends of the latter and then trim 

 the mesenteries close to the alimentary canal, removing the latter 

 from the body. This will leave the main blood-vascular trunks 

 so that their position may be recognized later. 



Split the alimentary canal lengthwise, wash it out and sketch 

 the character of its walls in the different regions. Are the transi- 

 tions from one to the other abrupt or gradual? 



Urogenital Organs. In the dorsal wall of the metacoele, after 

 removal of the other viscera see the mesonephroi or Wolffian 

 bodies (so-called kidneys), a pair of low ridges one on either side of 

 the attachment of the mesentery and extending nearly the length 

 of the body cavity. Note the short renal arteries passing from 

 the middle line into each mesonophros, adding them to the sketch 

 of the arterial circulation. The other urogenital structures vary 

 with the sex. 



Male. The testes are a pair of small flattened lobes dorsal 

 to the anterior part of the liver, each attached to the body wall 

 by a mesentery-like membrane, the mesorchium. Is this attached 

 lateral or medial to the mesonephros? 



Open the cloaca by inserting the scissors in the anus, and 

 carry the incision forward into the rectal region. In the surface 

 thus exposed note the sudden transition from the smooth lining 

 of the cloaca to the plicate condition in the rectum. 



On either side in the posterior part of the cloaca find a small 

 opening, the abdominal pore. Probe it; where does the probe 

 appear? 



In the dorsal wall of the cloaca is a prominent urogenital 

 papilla with an opening at its tip. Cut off the tip of the papilla 

 and with probe and scissors follow the tube forward, noting that 

 at first it is a simple cavity, the urogenital sinus, which branches 

 in front into two tubes, the Wolffian ducts, which can be traced 

 forward on the ventral surface of the mesonephros. (Behind, the 

 duct is straight; farther in front, tortuous. It receives tubules 

 from the mesonephros and others, indirectly, from the testis. 

 These points are not readily made out by the student.) 



Female. The ovaries are attached to the dorsal surface of the 

 ccelom, dorsal to the liver, by thin membranes, the mesovaria. 



