XIII 



PHYLUM CHORDATA 



117 



tail, is the very small emus (Fig. 758, a.), lying in a slight depres- 

 sion and having immediately behind it a small papilla pierced at 

 its extremity by the y/rinogenital aperture (z.). It has been sug- 

 gested that a pair of ridges, lying one on each side of the anus, 

 represent vestiges of pelvic fins ; otherwise there is no trace of 

 paired appendages. Two dorsal fins and a caudal fin are present, 

 the second dorsal being continuous with the caudal. 



Lampreys live on small Crustacea, Worms, and other aquatic 

 organisms, but also 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 ex- 

 panding and contracting like the thorax of a Mammal. The 



olf.c 



Nu* 



br.b.i br.b.s brb.a I (-* 



l.c.1 



alat.c 



l.C.3 



FIG. 750. 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. lot. c. 

 anterior lateral cartilage ; a;>. r. annular cartilage ; aw. c. auditory capsule ; br. b. 17, verti- 

 cal bars of branchial basket ; br. d. 1 7, external branchial clefts ; en. c. coruual cartilage ; 

 c/-. ,. cranial roof ; I. c. lU, longitudinal bars of branchial basket ; l<>. c. lingual cartilage ; 

 m. v. c. median ventral cartilage ; na. ap. nasal aperture ; nch. notochord ; Ni: 2, foramen for 

 optic nerve ; olf. c. olfactory capsule ; pc. c. pericardia! cartilage ; p. d. c. posterior dorsal 

 cartilage ; p. lat. c. posterior lateral cartilage ; */>. oc. a. sub-ocular arch ; st. p, styloid process ; 

 stii. r. styliform cartilage ; t. teeth. (After W. K. Parker.) 



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. 

 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 double lateral line and of minute pits on the 

 head. There is no trace of exoskeleton. 



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

 There is a persistent notochord (Fig. 750, 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 and bounding the spinal 



