398 CHARACTERS OF INVERTEBRATE ANIMALS 



The most remarkable character is found in the antennae, 

 which are branched. No breathing organs have, so far, been 

 discovered. The type-genus is Pauropus. 



CLASS 4. PRIMITIVE TRACHEATES (PROTOTRACHEATA) 



There are certain small groups of animals to which special 

 interest attaches on account of the speculations regarding gene- 

 alogy and origin of organs to which they have given rise. One 

 such group is the Hemichorda, already briefly described (p. 300); 

 another is the class now to be dealt with. It includes but a single 

 genus, Peripatus, a primitive type which appears to be of great 

 antiquity, and as the name of the class indicates, is supposed to 

 represent the ancestral stock from which the air-breathing arthro- 

 pods already described have been derived. The word ''represent" 

 must, however, as in such cases generally, be taken with some 

 qualification, for Peripatus can only be regarded as representing 

 that stock in a general sort of way, as no doubt it has to some 

 extent specialized on lines of its own, acquiring peculiarities which 

 adapt it to a special mode of life. To those who are not professed 

 zoologists it may appear that specialists make an unnecessary 

 fuss about an obscure creature that may briefly be described as 

 a "permanent caterpillar"; but the marvel is explained when we 

 remember that this lowly animal enables us to throw light upon 

 the origin and relationships of Myriapods, Arachnids, and Insects, 

 the last of which, taken by themselves, include the majority of 

 terrestrial species. 



Like many archaic forms, Peripatus has an extremely wide 

 geographical distribution, and its included species are found in 

 South Africa, the Malay Peninsula, East Australia, New Zealand, 

 South America, Central America, and the West Indies. Yet 

 all these widely - separated species resemble one another with 

 sufficient closeness to be placed in the same genus. 



Peripatus was first described (in 1826) as a Mollusc, later as 

 a Myriapod and as an Annelid (segmented worm). The late 

 Professor Moseley, in 1874, proved it to be an Arthropod, and our 

 detailed knowledge of its structure and development is mainly due 

 to the investigations of the late Professor F. Maitland Balfour and 

 Mr. Adam Sedgwick. It is now a widely-accepted view that 

 Arthropods have been derived from Annelids, and the special 



