88 



BRANCH VERTEBRATA. 



CLASS CYCLOSTOMA (si klo sto'ma). 



The CIRCULAR-MOUTHED VERTEBRATES have a cartilagi- 

 nous skeleton, naked skin, and no lower jaw. It is here 

 that we first meet with organs of special sense, con- 

 structed on the same plan as those of man, well-de- 

 veloped eyes, internal ears, and a nasal sac. 



ORDER HYPEROARTIA (hi per o ar'ti a). 



The Lamprey (lam'pre) is found in both salt and fresh 

 water. Its mouth is simply a circular sucking-cup, armed 



FIG. 150. 



FIG 151. 



Pe trom' ty zon mari'nus. Lamprey. JV, Nasal orifice. 



with numerous teeth. The tongue, working like a piston, 

 produces a vacuum by which the animal adheres to any 



object. If attached to a fish, its wasp- 

 like teeth enable it to scrape off the 

 flesh for food ; if to a stone, assisted 

 by the current, it can drag away quite 

 large ones from the spot it has chosen 

 for its nest. Circulation is effected by 

 means of a heart, the impure blood 

 from the body entering a single auricle 

 (a'ri ki), and from thence being poured into a muscular 

 ventricle, which forces it over the body by way of the 

 gills. These are bathed by water, which may enter 



Mouth of Lamprey. 



