1890.] 



THE YOUNG NATURALIST. 



13 



as keenly as it does to day, and that favorable modifications would 

 tend to become emphasized and perpetuated, and that under the same 

 natural laws which we can now observe, all existing organisms from 

 the simple amoeba to the highly specialized mammal have descended 

 from those ancient simpler forms. Without going back to the primi- 

 tive protists, we may start with a marine worm possessing a water 

 vascular system, for it is certain that all Arthropods are descended 

 from worm-like ancestors. The simplest form of the water-vascular 

 system consists of a main vessel on either side of the body, with 

 smaller transverse vessels connecting them, and the action of the 

 oxygen, contained in the water of these vessels, upon the blood of the 

 animal is similar to that in the gills of fishes. 



Descendants of some such marine worm became terrestrial, and 

 with only very slight structural modifications, the same vessels ceased 

 to contain water and allowed the admission of air, accompanied by a 

 corresponding increase of activity of the animals. There still exists a 

 genus which strikingly resembles this stage of the development of the 

 insect type, viz., Peripatus. These animals are true tracheal breathers 

 whose form reminds one of both the centipede and the caterpillar, and 

 probably represents the common ancestor of both. This genus occurs 

 in hot and damp situations, and we can easily imagine circumstances 

 under which it might become advantageous for it to resume an a- 

 quatic life. Having become more highly differentiated than its marine 

 ancestor, it could not resume a purely aquatic life without developing 

 special respiratory organs. It is probable that descendants of this 

 ancient pevipatus-like like form varied in different directions but the 

 variations which became permanent must all have presented one fund- 

 amental similarity, viz., that they were out-pushings of the integument, 

 communicating with the tracheal system ; not containing blood like 

 the gills of fishes but projecting from the body of the animal into the 

 water, the oxygen contained in which, readily passed through the thin 

 integument and thence into the tracheal system. Some of these vari- 

 ations are retained in the aquatic larvae of insects of to day ; in some 

 cases the projecting processes were filamentary, in others foliaceous. 

 It will be evident that an animal breathing by a tracheal system, the 

 air for which was obtained through dorsal foliaceous appendages would 

 benefit if those appendages could be moved in the water. As a matter 

 of fact in many existing aquatic larvae the projecting processes are so 

 moved ; moving respiratory organs accompanied by general increased 



