CHAPTER III 



PETROMYZON, A CHORD ATE WITH A SKULL, HEART, AND KIDNEY 



Externals. — Petromyzon, commonly known as the lamprey, is 

 an elongated animal not unlike a fish, but without paired fins 

 or jaws. Some species live in fresh water, and others in the 

 sea. Their length varies from a few inches to about four feet. 



The slimy epidermis is about a dozen cells in thickness and 

 contains glands. In the middle line the skin is produced into 

 median fins, two on the back and one round the tail. These 

 fins are stiffened only by rays of cartilage. 



At the front there is a circular mouth surrounded by horny 

 teeth. Behind the mouth on each side is a small deep-set eye, 

 and then seven apertures in a row. These are the external 

 openings of the gill-pouches. Dorsally, in the middle line 

 near the front, there is a small hole which is the single median 

 opening of the nasal sacs and the hypophysial cavity (see 

 p. 401). The anus is in the mid- ventral line, not far in front 

 of the ventral portion of the tail-fin. 



Through the mouth there protrudes a rasping organ called 

 the tongue, which like the sides of the mouth is covered with 

 horny teeth. These teeth are little cones, formed from the 

 ectoderm, and replaced from underneath when worn away. 

 They must be carefully distinguished from the teeth of all 

 higher forms, which are of a different nature. 



The lamprey fastens itself by means of its circular and 

 sucker-like mouth onto its food (mostly fish), and rasps at it 

 with its tongue. This method of feeding is very specialised 

 and almost degenerate ; and it has brought about several 

 specialisations in the structure of the animal. The horny 

 teeth are one of these. 



