Synthetic Types 75 



cipiently three-chambered, but the partition dividing 

 the auricle is not complete. In an ordinary fish the 

 heart contains only impure blood; in the Mudfish 

 some pure blood is brought to the heart, as well as 

 impure blood ; so is it also in Amphibians, though the 

 heart of the larva — e.g., the tadpole of a frog — is 

 for a while two-chambered just like that of an ordi- 

 nary fish. These instances might be multiplied, but 

 they may serve to show how the Dipnoi or Double- 

 Breathers (with lungs as well as gills) are inter- 

 mediate between ordinary fishes and the Amphibians. 

 They are annectent types. One must not jump to the 

 conclusion that the Dipnoi of to-day or of any day 

 are necessarily ancestral to the modern Amphibians, 

 but it is certain that they represent a grade of organi- 

 zation through which the ancestors of modern Am- 

 phibians must have passed. 



Of great evolutionary interest are the "synthetic 

 types," as they have been called, which combine in 

 their organization some of the characteristics of two 

 quite different groups of animals. One of the best 

 illustrations is Peripatus, an archaic animal some- 

 what like a permanent caterpillar, distantly related 

 to insects and distantly related to worms. Along with 

 some other genera it is included in the class Ony- 

 chophora (claw-bearers) or Prototracheata. They 

 are old-established animals, survivors of an ancient 

 fauna, and an index to their antiquity may be found 

 in their extraordinarily wide geographical distribu- 

 tion. They have had time to establish themselves in 

 many different parts of the world, such as Tropical 



