DOGFISH SHARK - Squalus acanthias

* PLACE PAPER TOWELS IN THE DISSECTING TRAY BEFORE YOU BEGIN !

External structure. Review the following external features:

gill slits; anterior and posterior dorsal fins; caudal fin; pectoral and pelvic fins; lateral line. Note the teeth. There are several rows of teeth along the margins of the jaws. The functional teeth are regularly being replaced by the interior reserve teeth.

Food passes from the mouth through a short esophagus to the long J-shaped stomach. A large triangular spleen is located at the bend of the stomach. The large, anterior part of the stomach is the cardiac portion of the stomach and the narrow, thick-walled posterior part is the pyloric portion. A distinct constriction marks the pylorus, the valve at the end of the stomach. Next food passes into the intestine which leads to the cloaca (the opening between the two pelvic fins.) Note the elongate rectal gland dorsal to the small intestine. This gland removes excess salt from the shark's body.

Note the large liver. The right and left lobes are elongated and fill much of the cavity. The median lobe is small and contains the gall bladder. The pancreas consists of a flattened ventral lobe that partially covers the duodenum, and an elongated dorsal lobe that extends to the spleen.

The kidneys are tan, narrow organs dorsal to the peritoneum, just to either side of the mid-line.

The gonads are located in the anterior part of the pleuroperitoneal cavity. In the male, the testis is attached to the body wall by a mesentery. Sperm pass through a duct into the kidney and then out of the body.

The size of the ovary depends on the reproductive state of the female. Just prior to ovulation, large egg cells will make the ovary quite large. The eggs enter the cavity, then enter the oviducts. The enlargement at the posterior end of the oviducts is the uterus.

Observe the heart located in the anterior end of the body cavity. Note the two chambers, the atrium and the ventricle. Locate the veins that bring blood back into the heart.

The large artery that carries blood along the spine of the shark is the Dorsal Aorta. A number of unpaired arteries branch off of it to carry blood to various parts of the body. The gastric branch carries blood to the stomach, a hepatic branch to the liver, and an intestinal branches to the small intestine. Some distance posteriorly, two branches arise close together. The anterior one is the anterior mesentaeric artery.

The other branch is the gastrosplenic artery which carries blood to the stomach, spleen, and pancreas. Locate the hepatic portal vein which conducts blood to the liver.

* PLACE YOUR SHARK IN THE PLASTIC BAG PROVIDED.

* CLEAN UP YOUR TRAYS AND TABLES.