272 



SYSTEMATIC ZOOLOGY 



FIG. 283. Petromyzon mart- 

 inis, lamprey eel. Head seen 

 from below, showing horny 

 teeth of buccal cavity. ( From 

 Claus and Sedgwick's Text- 

 book.) 



the head a single opening on the median line, the nasal aperture. 

 Along each side of the body, posterior to the eyes, is a row of 



seven gill slits. 



There is a well-developed notochord, and 

 in the region of the neural tube are seg- 

 men tally arranged cartilages, which corre- 

 spond to vertebras. The rest of the skeleton 

 consists of a cartilaginous brain case or 

 cranium, numerous cartilages in the head 

 and funnel, and a complicated series sup- 

 porting the gills and called the branchial 

 basket. There is a good-sized mouth cav- 

 ity, which communicates posteriorly with 

 two tubes, a ventral, the respiratory tube, 

 which is closed posteriorly and whose walls 

 are pierced by the gill slits, and a dorsal, 

 the oesophagus, which opens into a slight 

 enlargement, the stomach (Fig. 284). Con- 

 nected with the stomach is a single-lobed 

 liver. The intestine is a relatively straight tube with a posterior 

 enlargement, the rectum terminating in the anus. The heart 

 lies just posterior to the gills. The large body-cavity contains, 

 in addition to the alimentary canal and the liver, the kidneys, a 

 pair of band-shaped, elongated structures, and the single, very 

 large germ-gland. The ova or spermatozoa are discharged into 

 the body-cavity and pass to the outside through two pores near 

 the anus; these are called the abdominal pores. The muscles 

 of the body are arranged in myomeres. In the course of develop- 

 ment a larva is formed, called Ammocoetes ; its eyes are very 

 rudimentary and concealed beneath the skin. 



The lamprey feeds on various small aquatic animals and some- 

 times attaches itself by its suckerlike funnel to the bodies of 

 fishes, and then scrapes away the flesh by means of its tongue. 

 Lampreys were considered a great table delicacy by the Ro- 

 mans, and they arc still largely eaten in parts of Europe and 

 in some parts of America. The Germans call them " nine- 

 eyes." 



The hag-fishes or slime eels differ from the lampreys chiefly 

 in the almost entire absence of dorsal fins and in the presence 



