Phylum Chordata 571 



Suborder 1. Petromyzontia. Nasal aperture dorsal, having no connection 

 with the mouth, with seven pairs of gill pouches and well-developed 

 branchial basket. Examples : lamprey eel, Petromyson; the brook lamprey, 

 Ichthyomyson. 



Suborder 2. Myxinoidea. Nasal apertures terminal, having duct to pharynx, 

 branchial basket poorly developed, buccal funnel lacking, without larval 

 stage, with tentacles around mouth. Examples: hagfish, Myxine, along 

 the Atlantic; and Epiatrctus of the Pacific. 



Anatomy of Petromyzon marinus, the Sea Lamprey. — P. mar- 

 inus is common along both coasts of the United States and has recently 

 extended its range into the Great Lakes. It is a slender, cylindrical, gray 

 animal about 3 feet in length (Fig. 193, b). In the posterior region, it is 

 laterally compressed ; dorsally there are two fins and caudally one. At 

 the anterior end, the buccal funnel is margined by soft papillae and con- 

 tains horny teeth. The mouth is at the apex of this funnel. A tongue 

 provided with teeth is just below the mouth. The eyes are located on 

 the dorsal surface, and between them is the single nasal aperture. Pos- 

 terior to the nasal opening is the pineal eye which is covered by skin. 

 Just posterior to the eyes are the seven pairs of gill slits. On each 

 side of the body is a row of lateral sense organs, the lateral line. At 

 the posterior end, on the ventral surface is the anus; posterior to it is 

 the urogenital papilla with its common opening for the urinary and re- 

 productive systems. 



The cartilaginous skeleton is quite limited in its development. It 

 consists of the notochord which serves as the axial skeleton, the com- 

 plex skull, the cartilage of the tongue, a ring of cartilage surrounding 

 the buccal funnel, the paired visceral arches in the gill region, and small 

 arches dorsal to the notochord. The muscles of the body wall are 

 segmentally arranged in a 2 manner similar to that found in fish. The 

 tongue and buccal funnel are also well provided with muscles. 



Leading from the mouth is the short pharynx. From the pharynx 

 the digestive tract divides into two tubes: a dorsal esophagus and a 

 ventral respiratory tube which is guarded by a special fold, the velum. 

 The esophagus opens directly into the intestine, for there is no stomach. 

 The digestive and absorptive surface of the intestine is increased by the 

 presence of an inner longitudinal fold, a typhlosole or spiral valve. The 

 intestine terminates in the anus. There is but a single digestive gland 

 which is termed a "liver," but it is without a bile duct. 



