10 



the urinary papilla to open the cavity within it, the urinary sinus. 

 The connection of this with the pore at the tip of the papilla should 

 be demonstrated. Extend the incision forward. The urinary sinus 

 divides into right and left cornua which are of considerable size and 

 lie dorsal to the oviducts. Trace the Wolffian duct to the urinary 

 sinus and demonstrate its opening into the cornu anterior to the 

 point where the two cornua unite. 



In young specimens the ovaries are small, and the oviducts are 

 narrow, white tubes lying along the medial margins of the kidneys. 



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 ovi- 

 duct 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 urogenital sinus. 



RESPIRATORY ORGANS 



Open the anterior gill-pouch of the left side by dorsal and ven- 

 tral 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 pre- 

 vent 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 demi branch. If the specimen is injected a large blood 

 vessel can be seen through the skin in the inner border of the demi- 

 branch, 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 num- 

 ber of the demi-branchs 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.* 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 Euga- 

 leus 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. 



*See footnote, p. 31. 



