xiii PHYLUM CHOHDATA 125 



eyes. On the ventral surface, marking the junction between trunk 

 and tail, is the very small anus (Fig. 802, a.), lying in a slight depres- 

 sion, and having immediately behind it a small papilla pierced at 

 its extremity by the urinogenital aperture (z.). There is no trace 

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

 unpaired fins. Two dorsal fins of approximately equal dimen- 

 sions, 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 ex- 

 panding and contracting 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 silverj' 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 lateral line which is superficial, not enclosed 

 in a canal, 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. 794, 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 (w. a.) arranged segmentally, bounding the spinal canal 

 on each side, and corresponding to rudimentary neural and inter- 

 neural arches. For the rest of its extent the spinal canal is 

 enclosed only by tough, pigmented connective-tissue. Slender 

 rods of cartilage support the median fins. 



The cranium also exhibits a very primitive type of structure. 

 Its floor is formed by a basal plate (Fig. 795, b. pi.), made by the 

 union of the parachordals and trabeculse, and surrounding pos- 

 teriorly the fore-end of the notochord. Immediately in front of 

 the termination of the notochord is a large aperture, the basi- 

 cranial fontanelle (b. cr.f.), due to the non-union of the posterior 

 ends of the trabeculaj; through it passes the pituitary pouch, pre- 

 sently to be referred to (Fig. 798), on its way from the olfactory sac 

 to the ventral surface of the notochord. Lateral walls extend 

 upwards from each side of the basal plate, but the roof of the 

 cranium is formed by membrane except at one point, where a 

 narrow transverse bar (cr. r.) extends across between the side-walls 

 and furnishes a rudimentary roof United with the posterior end 



