446 ZOOLOGY 



sufficient water, but it has been determined that many 

 of them require the presence of certain kinds of fishes. 

 They produce a larval form called the glochidium. 

 The glochidia attach themselves to the gills of particular 

 fishes, where they become covered or encysted (in the 

 manner of a gall), and when at length sufficiently de- 

 veloped they break away and resume independent life. 

 Thus the mussel Lampsilis luteolus requires the presence 

 of basses or perches ; Oboraria ellipsis is temporarily 

 parasitic on the sturgeon. One species, Hemilastena 

 ambigua, infests the gills of an amphibian, the Necturus 

 or mud puppy. 



Distribution 6. The study of distribution may be made to throw 

 geography * much light on the past history of the earth. Thus, the 

 of the past marine fishes on the two sides of the Isthmus of Panama 

 are so much alike that we are quite sure that the Isthmus 

 was formerly submerged. This is now confirmed by the 

 discovery of many fossil sea shells in the course of dig- 

 ging the canal. When islands, such as the British Isles, 

 have a biota 1 nearly identical with that of the neighbor- 

 ing continent, we infer land connections in the past. 

 Oceanic islands, which were never connected with any 

 mainland, have only creatures of a type which might 

 have crossed the sea. For example, they never have 

 any truly native frogs, since these animals cannot en- 

 dure salt water. When we are sure that two lands 

 have formerly been united or separated, the degree of 

 resemblance in the products is an index to the length, of 

 time which has elapsed since the change to present 

 conditions occurred. 



1 Biota = fauna + flora; the total life of the country. 



