GENERAL STUDY OF TYPICAL CHORD ATES 15 



The anterior end of the head presents a peculiar appearance, owing to the 

 apparent absence of a lower jaw (the jaw is probably present in a degenerate 

 condition) and the presence on the ventral side of the head of a large bowl- 

 shaped depression, the buccal funnel. The edges of the buccal funnel are pro- 

 vided with soft papillae, and its interior is studded with brown, horny teeth, 

 definitely arranged. At the bottom of the funnel is a projection, called the 

 tongue, also covered with teeth. The mouth is just dorsal to the tongue. Lamp- 

 reys attach themselves to the bodies of fishes by means of the buccal funnel and 

 rasp off their flesh by filing movements of the tongue. 



On the dorsal surface of the head is an opening, the nasal aperture or nostril, 

 which leads into the olfactory sac. Cyclostomes differ from other vertebrates 

 in possessing but one nostril, although this is probably a secondary condition. 

 On each side of the head on a level with the olfactory organ is an eye, not provided 

 with eyelids. Posterior to the eye on each side of the head is a row of seven 

 oval openings, the gill slits. 



Along the sides of the body the boundaries of the myotomes are generally 

 noticeable through the skin. In the median ventral line at the junction of trunk 

 and tail will be found a pit. In the anterior end of this pit is the anal opening 

 and immediately behind the anus a projecting papilla, the urogenital papilla, 

 which receives the products of the kidneys and reproductive organs and dis- 

 charges them to the exterior through an opening at its tip, the urogenital 

 aperture. 



2. Sagittal section of the anterior end. — Make a median sagittal section of 

 your specimen to a point about an inch posterior to the last gill slit or study a 

 section so prepared. Examine the cut surface and identify the following: 



a) Digestive tract: Observe again the buccal funnel with its teeth and tongue. 

 Note the large muscle masses extending posteriorly from the tongue, by means 

 of which the rasping movements of the tongue are brought about. Find the 

 mouth opening above the tongue and follow it into a passage, the buccal cavity 

 which slopes ventrally. The buccal cavity opens at its posterior end into two 

 tubes, an upper smaller one, the esophagus, and a larger ventral one, the pharynx, 

 the wall of which is pierced by seven oval openings. A fold, the velum, is present 

 at the entrance of the buccal cavity into the pharynx. The esophagus leads 

 into the remainder of the digestive tract, but the pharynx, which in the 

 embryo constituted the anterior part of the digestive tract, ends blindly in 

 the adult. 



b) Respiratory system: The seven openings in the wall of the pharynx are 

 the internal gill slits. They open into much enlarged gill pouches which bear 

 the gills on their walls and which communicate with the exterior through the 

 external gill slits already noted. Probe into one of the gill pouches and note the 

 leaves or gill lamellae borne on its walls. The dorsal portions of the gill pouches 

 inclose the narrow esophagus between them. 



