326 ZOOLOGY 



through which the water passes, eventually escaping 

 through a median aperture. The mouth, surrounded by 

 long, whiskerlike cirri, is on the under surface, and is 

 without jaws. The alimentary canal possesses a 

 diverticulum or sac which constitutes a primitive liver, 

 and represents the stage of development of that organ 

 found in the early embryos of vertebrates. There are 

 pigment spots on the dorsal nerve cord which appear 

 to be primitive organs of vision, while a pit at the an- 

 terior end seems to represent the beginning of an organ 

 of smell. Finally, the muscular tissue of the animal 

 is segmented (the divisions are called "myotonies"), 

 apparently the beginning of that segmentation which 

 in vertebrates finds expression in the vertebrae with 

 their attached ribs. Thus it seems that the intercostal 

 muscles, which in ourselves lie between the ribs and 

 serve to expand the chest in breathing, are actually 

 more primitive than the ribs supporting them. The 

 Amphioxus, therefore, though not a vertebrate, repre- 

 sents a very remarkable approach to the vertebrate 

 type, and does not show the so-called degenerate fea- 

 tures of the other prochordates. 



Cycio- 6. The next stage in evolution, so far as known to us, 



** ^ s represented by the Cyclostomes ("round-mouths"), 



andhag- including the lampreys and hagfishes. These are not 

 prochordates, neither are they true fishes. They pos- 

 sess a primitive but genuine brain, with a cartilagi- 

 nous skull. The notochord is enveloped in a sheath, 

 but there are no distinct vertebrae. There are paired 

 eyes, but the nostril is single and median. There is 

 no lower jaw, and there is no trace of paired fins. The 

 liver is of the same general type as that of vertebrates 

 in general. Lampreys live in the sea or in fresh water, 

 and feed on the flesh and blood of fishes. The peculiar 



