BREATHING MECHANISM OF THE LAMPREYS. II 



of the water tube at its junction with the pharynx. They are 

 united to one another in the middle line for I or 2 mm. near their 

 dorsal ends but are free elsewhere, so that there is left between 

 them a slit-like opening. They are concave caudally and their 

 lateral edges are united to the water tube. They function nor- 

 mally to prevent water entering the pharynx from the water tube. 

 In order to understand the relation between the velar jaws and 

 the velar valves the cartilaginous frame-work common to the two 

 must be carefully dissected out. It is found to bear a striking 

 resemblance to a pair of mechanic's nippers except that it is in 

 one piece, whereas the latter is in two pieces. The jaws of this 

 nipper-shaped cartilage are much smaller than the handles and 

 have a much smaller arc between them. In fact the proportion 

 between the jaws and handles of the cartilaginous apparatus is 

 much the same as is found in the manufactured 

 tool and like it tends to power in grasping. Ap- 

 proximation of the handles of a pair of nippers 

 brings the jaws together. This is due to the fact 

 that the halves of the nippers are crossed and 

 fastened together by a pin. The two halves of 

 the nipper-shaped cartilage are not crossed, hence 

 when the handle is pushed inward, the corre- 

 sponding jaw moves outward. The jaws of these 

 nipper-like cartilages form the supporting skele- 

 ton of the velar jaws found in the pharynx, while p- IG 7 Carti- 

 the handles extend into the lateral walls of the laginous skeleton 

 water tube as slender cartilages which end on its the ve Jaws 



and valves. 



ventral wall just cranial of the first internal gill 



opening (Fig. 6). The cartilages lie along the lateral attached 



edges of the velvar valves and support them. 



There are three pairs of muscles which are inserted into the 

 nipper-shaped cartilage common to the velar jaws and velar valves. 

 These are the velo-pharyngeus, velo-hyomandibularis internus, 

 and the velo-hyomandibularis externus. They are sufficiently 

 described by Furbringer (1875), although he does not mention the 

 velar jaws. Their combined action is to move the cartilaginous 

 rods supporting the velar valves (the handles of the nippers) in- 

 ward and outward. When the rods are moved inward, toward 



