88 VERTEBRATES WITHOUT JAWS iv. 6 



6. Alimentary canal of lampreys 



The sucker is bounded at the edges by a series of lips, which besides 

 being sensory serve also to make a tight attachment when the lamprey 

 sucks (Fig. 52). In the sucker are numerous teeth, whose arrangement 

 varies in the different types of lamprey. These teeth are horny epi- 

 dermal thickenings, supported by cartilaginous pads, and are there- 

 fore not comparable with the teeth of vertebrates, which are derived 





Fig. 52. Sucker of Petromyzon show- Fig. 53. Section through tooth of lamprey 



ing outer circular lip, teeth, and tongue, 1, horny cap; 2, stellate tissue; 3, cap to replace 



with special teeth, at the centre. , ; 4> connective tissue; 5, epidermis of mouth; 



(After Parker.) 6, cartilage; 7, proliferative layers of epidermis 



that produce the horny cells. 

 (After Hansen, from Kukenthal.) 



mainly from mesodermal tissues (Fig. 53). The sharper and larger 

 teeth are borne on a movable tongue, which is used as a rasp (Fig. 54). 

 An annular muscle runs round just above the lips of the sucker and 

 presumably serves to narrow the margin and hence to release the fish. 

 The remaining muscles are mostly attached to the tongue and base 

 of the sucker. The largest of these muscles, the m. cardioapicalis, is 

 attached posteriorly to the cartilage surrounding the heart and in front 

 is prolonged into a conspicuous lingual tendon, which is attached to 

 the tongue and serves to pull it backwards. Presumably the action of 

 this muscle deepens the oral cavity and is thus the main agent securing 

 attachment of the sucker. There is a collar of circular fibres around 

 the front end of the cardio-apical muscle, serving to lock the tendon 

 and maintain the suction. Dorsal and ventral to the main tendon are 



