8 4 VERTEBRATES WITHOUT JAWS iv. 4- 



microphagous: the adult lamprey has a sucking mouth, and usually 

 lives in the sea, where it feeds on other fishes. Lampetra the lamprey 

 (Fig. 47), is a typical example, common in Great Britain. The adult 

 is an eel-like animal about 30 cm long, black on the back, and white 

 below. The surface is smooth, with no scales. The skin is many- 

 layered (Fig. 48). The outermost cells have a striated cuticular border. 

 Mixed with these epithelial cells the lamprey, like most aquatic 

 vertebrates, has many gland-cells for producing slime. Below the 

 epidermis lies the dermis, a layer of bundles of collagen and elastin 



Fig. 47. Brook lampreys, Lampetra planer i. 



A, ripe female, with anal fin; B, ripe male; note shape of dorsal fin and presence of 



copulatory papilla. (Curves due to fixation.) 



fibres, running mostly in a circular direction. This tissue is sharply 

 marked off from a layer of subcutaneous tissue containing blood- 

 vessels and fat, as well as connective tissue. There are pigment cells 

 in the dermis and a thick layer of them at the boundary of dermis and 

 subcutaneous tissue. The chromatophores are star-shaped cells whose 

 pigment is able to migrate, making the animal dark or pale. This 

 change is especially marked in the larva and is produced by variation 

 in the amount of a pituitary secretion (p. 107). 



The head of the lamprey bears a pair of eyes and a conspicuous 

 round sucker. On the dorsal side is a single nasal opening, and behind 

 this there is a gap in the pigment layers of the skin through which the 

 third or pineal eye can be seen as a yellow spot. There are seven pairs 

 of round gill openings, which, with the true eyes (and some miscount- 

 ing or perhaps inclusion of the nasal papilla), are responsible for the 

 familiar name 'nine eyes'. There is no trace of any paired fins, but the 

 tail bears a median fin, which is expanded in front as a dorsal fin. 

 There are sex differences in the shape of the dorsal fins of mature 

 individuals and the female has a considerable anal fin (Fig. 47). 



