172 Dynamic Theory. 



by them so as to allow of the transfer of tli elements by osmosis 

 through the delicate membranes of the vessels. The difference between 

 aquatic and aerial respiration is one of degree only, the essential princi- 

 ples and end being precisely the same. But it is obvious that an 

 animal breathing air directly will obtain more oxygen than one breath- 

 ing it through water, and that since the energy liberated for the heat 

 and activity of the animal is in proportion to the amount of carbon oxi- 

 dized, it follows that the air-breather will be superior in force and activity 

 to the water-breather, other things being equal. And conversely those 

 animals which are compelled b}^ their environment to use greater activity 

 will of necessity seek the more abundant supply of oxygen furnished by 

 the atmosphere. Hence we find in every class of gill-breathing animals, 

 some members of the class whose gills are modified to enable them to 

 take in air directly, instead of depending on the water alone as a me- 

 dium. This modification in general consists of an enlargement of the gill 

 cavities, and the lining or filling the enlargement with folds of mucous 

 membrane]; the new parts being adapted to direct contact with the air, 

 while the gill cavities with the old fashioned gills for water-breathing 

 still remain. This process of enlargement is really nothing more than 

 a continuation of the original modification by which gills were first 

 formed, they being nothing more than folds of the skin subjected to the 

 special function of respiration. The animal possessing the supplement- 

 ary air-breathing caAdty is in reality an amphibian. The transitional 

 stage between those having water gills and those having these additional 

 air spaces is found in the gilled animals that can live where it is damp 

 merely. In their case the moisture in the air is sufficient to keep the 

 gills in proper mechanical condition to absorb the air. The land leeches 

 of India are examples of skin-breathing animals that by reason of the 

 moisture of the climate can live on land. Their whole skin is a respira- 

 tory surface and ordinarily they are aquatic, but they are extremely 

 numerous on land in India and on the Indian islands, where the skin is 

 kept moist by the saturated atmosphere. They are very numerous on 

 the trees, and drop down upon every thing passing under, so that other 

 animal life is impossible there. (Semper.) The Planarians and Nemer- 

 teans, aquatic worms that are also skin-breathers, are likewise able 

 to live in moist places on land. It is well known that many kinds of 

 fish can be carried about and kept alive on land for days if only care be 

 taken to keep their gills moist. The Neretina, a gasteropod aquatic 

 shell mollusk, has species that live mostly on land. Their gill cavities 

 contain a network of blood vessels which are united into one, that com- 

 municates with the auricle of the heart, an arrangement equivalent to 

 lungs for air breathing. In addition to this it has gills which it uses in 

 the water, where it goes to lay its eggs. Some species of Snails also 



