CYCLOSTOMATA 195 



Locomotion. The Amphioxus has a median dorsal fin which 

 extends over the tail both dorsally and ventrally. The tail, 

 that portion of the body posterior to the alimentary tube and 

 filled with muscle, is the chief organ of locomotion. It is noc- 

 turnal, swimming about at night, but quickly returns to its 

 burrow if disturbed. It can burrow in the sand with either head 

 or tail. 



Nervous System. A simple dorsal nerve lies above the noto- 

 chord. It does not reach entirely to the front of the body. Its 

 anterior tip is called the cerebral vesicle, in which there is an 

 eye-spot. There is also possibly an olfactory organ consisting 

 of a simple pit reaching from the skin down into the anterior 

 tip of the nerve cord. 



SUB-PHYLUM IV. CRANIA'TA OR VERTEBRA'TA 



These are chordates having a brain or skull. The group 

 includes fishes, amphibians, reptiles, birds, and mammals. 

 The body is usually elongated and more or less cylindric. 

 The mouth is situated anteriorly. Ventrally and near it, ex- 

 cept in Cyclostom'ata, are the paired nostrils. Situated in 

 the head there are also a pair of eyes and a pair of ears, 

 though the ears are not always external. Gill-slits are 

 never more than seven in number, and partially or alto- 

 gether disappear in the adult air-breathing forms. There 

 are one or two pairs of jointed limbs, but in some cases they 

 are rudimentary or wanting. 



CLASS I. CY'CLOSTOM'ATA 



The animals of this class inhabit both fresh and salt water. 

 They have no lower jaw. The mouth is suctorial, the skull 

 cartilaginous, the notochord persistent, and the teeth horny. 

 The neural arches are rudimentary. There are no limbs or 

 scales and no paired fins; but unpaired dorsal and caudal 

 ones are present. The class includes the lampreys (Fig. 

 157) or "lamprey-eels" and the hag-fishes. The skin is 



