rETROMYZON FLT^VIATILIS. 4-25 



eliance animal substances. In England it commonly reaches a length of 

 twelve or fifteeii inches; it is sometimes a few inches longer in Continental 

 countries. It has much the same aspect and Eel-like shape as the preceding 

 species, and is compressed at the tail (Fig. 207). 



The head is smaller than the anterior part of the body ; the greatest thick- 

 ness between the gill-openings is two-thirds of tlie height. The length of 

 the head to the last branchial aperture, is fully one-fifth of the length of the 

 Ush. The mouth is terminal and nearly circular, so placed as to look obliquely 

 downward and forward. It is surrounded by a short fringe of tentacles, within 

 which there is a row of very small teeth, which are scarcely visible, and easily 

 fall off. The arrangement of teeth in the oral disc is very 

 different from that in the Sea Lamprey, and the form of the "1' 



opening of the gullet, which contains the tongue, is quadrate 

 rather than circular (Fig. 208). 



At the back of the gullet there is a large plate similar to 

 that in the Sea Lamprey, only much less curved, and it 

 similarly contains about eight horny denticles. The corre- 

 sponding plate on the opposite side or front of the mouth 



'■ . . . 1-1 ^ '"• 208.— PETRO- 



similarly contains two teeth, but instead of being close myzon fluviati- 

 together, as in the Sea Lamprey, they are wide apart, at fuom"below. 

 the anterior corners of the mouth. There are no teeth at 

 all behind the posterior or mandibular tooth-plate ; and in front of the 

 maxillary tooth-plate there is only a crescent of about five little teeth 

 scarcely larger than those of the outer circle to which they are parallel. But 

 there are three tooth-plates arranged in a longitudinal row on each side of the 

 mouth. The middle plate on each side has three denticles arranged transversely. 

 The upper plates have two denticles, each transversely prolonging the line 

 of the maxillary teeth, and the lower plates have also two denticles pro- 

 longing the transverse line of the mandibular bar. The tongue within the 

 mouth has two teeth which are transverse, parallel to each other, and carry 

 many denticles. 



The eye has the same lateral position as in the Sea Lnmprey, but varies 

 in size from one-ninth to one-twelfth of the length of the head, reckoning to 

 the last branchial aperture. It is separated from the other eye by twice 

 its diameter, and in the young fish the distance is less. It is seven times its 

 diameter from the last gill-aperture. Between the eyes on the upper side of 

 the head is the single nasal aperture which extends forward as a short tube. 

 The gill-apertures are arranged in a longitudinal series, as in the Sea Lamprey, 

 with a valve in front and a serrated margin behind. They are about the 

 diameter of the eve from each other. 



