THE VERTEBRATA 14! 



sea-sand ; it lies buried in it, with its mouth 

 just uncovered. Its food consists of microscopic 

 vegetable organisms. Its distribution is very 

 wide ; it is found in both the Atlantic and Pa- 

 cific waters. It occurs most abundantly in the 

 salt-water lakes of Sicily, and in the Gulf of 

 Naples. The specimen first seen, in 1778, came 

 from the coast of Cornwall. There are eight 

 species; the one which is found in the English 

 Channel is the Amphioxus lanceolatum, also found 

 in the Mediterranean and on the shores of North 

 America. 



The classes of the Vertebrata are Fishes, 

 Amphibia, Reptiles, Birds and Mammals. We 

 used to learn that of these, fishes had gills, and 

 Amphibia gills for a time ; but, to be strictly 

 accurate, we must say that fishes have gills, and 

 all the rest of the Vertebrata have gills for a 

 time. There is no exception to this rule, not 

 even among the highest vertebrates of all. But 

 in those vertebrates which stand higher in the 

 scale of life than Amphibia, viz., Reptiles, Birds, 

 and Mammals, these gills are never brought into 

 use. They only exist in the early embryo, and 

 afterwards disappear, giving rise by their modifi- 

 cation to other structures. 



Strange to say, one of these structures is the 

 ear. This takes its origin from one of the gill- 

 " clefts" or spaces. The Eustachian tube, which 

 communicates between the ear and the nose, is 

 part of this cleft ; and the little bones which are 

 inside the ear represent the bones of that gill- 

 cleft. For, in fishes, bones support each gill and 

 are connected together to form a complex ar- 

 rangement. In the higher vertebrates, which 

 possess gills only in the embryo, this gill-skeleton 



