_Nephrostomes_, short, segmentally arranged kidney tubules which open to the coelom by a funicular aperture, are found by a close examination along the medial border of each kidney. They should be observed carefully with the aid of a good dissecting lens. Learn the significance of these structures. In the course of development two sets of nephridia (kidneys) are formed. The first (pronephros) develops just back of the head of the embryo, but does not persist in the adult. Its duct, known commonly as the Muellerian duct, develops into the functional oviduct of the female, but forms an apparently useless vestige in the male. The second kidney (mesonephros) develops behind the first and is the excretory organ of the adult. Its duct (frequently given the name of Wolffian duct) is the urinary duct in the female, but functions in the male chiefly as a sperm duct, and therefore is called the vas deferens. The collecting tubules of the posterior portion of the kidney of the male unite to form a urinary duct which opens into the Wolffian duct or the uro-genital sinus. RESPIRATORY ORGANS Open the anterior gill pouch of the left side by dorsal and ventral cuts extending from the angles of the cleft, but cutting only as far as is necessary to see the structures within the pouch. Upon the medial side the gill pouch opens into the pharynx by a dorso-ventral slit, guarded by projecting cartilagineous _gill rakers_, which prevent particles of food from passing into the gill pouch with the respiratory current. On both the anterior and posterior wall of the pouch is a _demibranch_. If the specimen is injected a large blood vessel can be seen through the skin in the inner border of the demibranch, and small vessels passing from this into the leaflets, where the interchange of gases between the water and blood takes place. Open similarly each pouch of the same side, observing the number of the demibranchs and their relation to the pouches. Upon the anterior wall of the spiracle demonstrate a row of small vestigial gills; being supplied with arterial instead of venous blood they form what is termed a _pseudobranch_. VASCULAR SYSTEM HEART AND VENTRAL AORTA. Continue the longitudinal incision through the skin as far as the mandible.[1] Dissect away the sheet of muscles between the gill pouches and the mandible, exposing a slender muscle which extends from the pectoral girdle to the middle of the mandible. The _thyroid gland_ lies dorsal to the anterior end of this muscle, close against the mandible. (The thyroid of Eugaleus is a broad, flattened structure covering the anterior ends of the coracohyoideus muscles.) Carefully dissect out the muscles lying between the branchial pouches of the right and left sides. Footnote 1: See footnote, p. 31.