of sensory organ, the pit organs, closely related genetically to the 

 sensory canals. Two longer lines of pit organs, (the mandibular 

 pit organs), will be found a short distance behind the mouth. Sim- 

 ilar pit organs are found in front of the endolymphatic pores, and 

 above the anterior part of the lateral line. 



Except for a few small areas the entire surface of the body is 

 covered with small, sharp-pointed denticles (placoid scales) . Each 

 consists of a diamond-shaped basal plate embedded in the dermis, 

 from which projects a leaf -like, backward directed spine. A piece 

 of skin should be removed and examined under a low magnifica- 

 tion. The dermis is so dense and pigmented that the basal plate is 

 not easily studied without further manipulation. For this purpose 

 boil a piece of skin in 5% caustic potash solution until it is softened, 

 but not till it disintegrates. Then clear it in glycerine. Examine 

 the individual denticles under a higher magnification. The den- 

 ticle consists of dentine, the spine being of a much denser structure 

 than the base. The teeth and the large spines of the fins and claspers 

 are also composed of dentine and may be considered as modified 

 placoid scales. Denticles, teeth, and spines are covered with a 

 shiny, enamel-like layer which, however, does not appear to be true 

 enamel such as covers the teeth of higher vertebrates. The shape 

 of the scales and their closeness vary on different regions of the 

 body, and there are certain regions entirely free from them, namely, 

 back of the dorsal, pectoral and pelvic fins, the medio-dorsal sur- 

 faces of the claspers, inside the upper lip and the labial pockets. 



DISSECTION OF THE ABDOMINAL VISCERA 



Place the dogfish on its back and, commencing at the middle of 

 the abdomen, make an incision through the body wall a quarter of 

 an inch to one side of the midline. Carry this forward to the pec- 

 toral girdle and backward through the pelvic girdle to the cloaca; 

 not, however, cutting the wall of the cloaca. 



A large vein, the lateral vein, runs along the inner surface of 

 each lateral wall of the abdomen. After identifying these, cut 

 through the body wall transversely on both sides of the abdomen at 

 the level of the posterior attachment of the pectoral fin as far as 

 the lateral vein. Turn the flaps outward and fasten. 



The coelom or body-cavity consists of two portions, the abdom- 

 inal and pericardial cavities. The abdominal cavity, which has now 

 been opened, extends from the pectoral girdle to the cloaca and 

 along the sides of the latter; it communicates with the exterior 

 through the abdominal pores on either side of the cloaca. Pass a 

 bristle or probe through each abdominal pore into the cloaca. 



Without dissecting, identify the following parts and observe 

 their relations: 



The peritoneum, the smooth lining of the body-wall, which is 

 reflected over the viscera. 



The liver, a large, gray organ attached anteriorly and almost 

 completely divided into two lobes which extend well back along 

 the sides of the abdominal cavity. 



The stomach, lying between the lobes of the liver. Its posterior 

 end is bent forward upon itself in the form of a U. The two limbs 

 of the stomach are known as the cardiac (proximal) and pyloric 

 (distal), respectively. With a second turn to the right and back- 

 ward it enters the intestine. 



The intestine, a large, thin-walled tube extending from the 

 stomach to the cloaca. 



