A HISTORY OF VERTEBRATES: FISHES 



429 



particles were somehow trapped in the pharynx. It seems hkely that the 

 ancestral vertebrates, like the lower chordates of the present day, were 

 filter feeders. 



196. Living Jawless Vertebrates 



Ostracoderms became extinct by the end of the Devonian, and the 

 living lampreys and hagfishes of the order Cyclostomata are a specialized 

 remnant of the class Agnatha (Figs. 22.4 and 22.5). They are jawless, 



Dorsal aorba — i 

 Esophagus — 

 Respiratory tube.—] 

 Internal gill slit 

 Sin as venosus 



Olfactory Sac 



Pineal eye. 

 Cranial cartilaoeS" 

 Brain 



Median riostril 

 rSinuS 



Mouth 



Horny 

 tce-th 



cal funnel 



of br^x2ichia] /, 



basket /Hypophyseal" 

 igual cartilage — pouch 

 and muscles 



Tongue 

 Buccal cavity 



Sinus 



Rudimentary vertebrae- 

 Spinal cord. 

 Notochord 

 Cardinal veiiv 

 Intestine 

 Liver 

 Pleuroperitoncal 

 cavity 

 Pericardial 

 cavity 

 Ventricle 

 Vcntrcil aorta 



InFerior 



jugular vein. 



Figure 22.4. A diagrammatic representation of the more important organs found 

 in the anterior part of the lamprey. 



have more gill slits than other living fishes, lack paired appendages, 

 retain a pineal eye, and have a single median nostril. liesides leading 

 to an olfactory sac, this nostril opens into an hypophyseal sac that 

 passes beneath the front of the brain. Much of the pituitary gland of 

 higher vertebrates is derived from an embryonic hypophysis. Cyclostomes 

 differ from ostracoderms in several respects: they have an eel-like shape 

 and a slimy, scaleless skin, and they are predators or scavengers. Many 

 lampreys, like the ostracoderms, live in fresh water, but some spend 

 their adult life in the ocean and return to fresh water only to reproduce. 

 The hagfishes are exclusively marine. 



A familiar example of the group is the sea lamprey, Petromyzon 

 marinus. The chief axial support for the body is a notochord which 

 persists throughotit life and is never replaced by vertebrae. Rudimentary 

 vertebrae are present, however, on each side of the notochord and 

 spinal cord. The brain is encased by a cartilaginous cranium, and the 

 gills are supported by a complex, cartilaginous lattice-work known as 

 the branchial basket, which appears to be homologous to the visceral 

 skeleton of other fishes. 



The mouth lies deep within a buccal funnel, a suction-cup mech- 

 anism with which the lamprey attaches to other fishes (Fig. 22.5). The 

 mobile tongue armed with horny "teeth" rasps away at the prey's flesh, 

 and the lamprey sucks in the blood and bits of tissue. It has special 

 oral glands that secrete an anticoagulant which enables the blood to 



