xni PHYLUM CHORDATA 121 



of paired appendages, and the only organs of locomotion are the 

 unpaired fins. Two dorsal fins of approximately equal dimensions, 

 separated by a notch, and a caudal fin are present, the second dorsal 

 being continuous with the caudal. 



Lampreys prey upon Fishes, attaching themselves to the bodies 

 of the latter by the sucker-like mouth, and rasping off their flesh 

 with the armed tongue. They are often found holding on to stones 

 by the buccal funnel, and under these circumstances perform regular 

 respiratory movements, the branchial region expanding and con-' 

 tracting like the thorax of a Mammal. The reason of this is that 

 when the animal is adhering by the mouth the respiratory current 

 cannot take its usual course entering at the mouth and leaving 

 by the gill-slits but is pumped by muscular action both into and 

 out of the branchial apertures. 



The skin is soft and slimy, mottled greenish-brown in P. marinus, 

 bluish above and silvery on the sides in the fresh- water species. 



nlj:c br.b.t br.b.i brh:, l-C-S 



sb.oc.a br.cl.1 f c + 



l.C.J 



FIG. 804. Petromyzon marinus. Skull, with branchial basket and anterior part of verte- 

 bral column. The cartilaginous parts are dotted, a. d. c. anterior dorsal cartilage ; a. lat. c. 

 anterior lateral cartilage ; an. c. annular cartilage ; au. c. auditory capsule ; br. b. 1 7, 

 vertical bars of branchial basket ; br. cl. 1 7, external branchial clefts ; en. c. cornual 

 cartilage ; cr. r. cranial roof ; 1. c. 1 4, longitudinal bars of branchial basket ; Ig. c. lingual 



subocular arch ; st. p. styloid process ; sty. c. styliform cartilage ; t. teeth. (After W. K. 

 Parker.) 



The epiderm contains unicellular glands, the secretion of which 

 gives its slimy character to the skin. The segmental sense-organs 

 take the form of a lateral line which is superficial, not enclosed in a 

 canal, and of minute pits on the head. There is no trace of exo- 

 skeleton. 



Skeleton. The axial skeleton of the trunk is very simple. 

 There is a persistent notochord (Fig. 804, nch.) with a tough sheath 

 composed of an inner fibrous and an outer elastic layer. Attached 

 to the sides of the notochord are little vertical rods of cartilage (n.a.), 

 arranged segmentally, bounding the spinal canal on each side, and 

 corresponding to rudimentary neural and interneural arches : in 

 the caudal region these fuse into a single plate perforated by 

 foramina for the spinal nerves and sending off processes to the base 



