Chap, vi.] 



PERII'ATUS CAPENSIS. 



137 



Peripatus is an animal of the very highest importance and 

 antiquity, and I believe it to be a nearly related representative 

 of the ancestor of all air-breathing Arthropoda, i.e., of all insects, 

 spiders, and Myriapods. 



The animal has the appearance of a black caterpillar, the 

 largest specimens being more than three inches in length, but 

 the majority smaller. A pair of simple horn-like antennae pro- 

 ject from the head, which is provided with a single pair of 

 small simple eyes. Beneath the head is the mouth provided 

 with tumid lips and within with a double pair of horny jaws. 

 There are seventeen pairs of short conical feet, provided each 



PERIPATUS CAPENSIS. (Natural size). 



with a pair of hooked claws. The skin is soft and flexible, and 

 not provided with any chitinous rings. 



The animal breathes air by means of tracheal tubes like 

 those of insects. These, instead of opening to the exterior by 

 a small number of apertures {stigmata) arranged at the sides of 

 the body in a regular manner as in all other animals provided 

 with tracheae, are much less highly specialized. The openings 

 of the short tracheae are scattered irregularly over the whole 

 surface of the skin. 



It appears probable that we have existing in Peripatus 

 almost the earliest 

 stage in the evolution 

 of tracheae, and that 

 these air tubes were 

 developed in the first 

 tracheate animal out 

 of skin glands scat- 

 tered all over the 

 body. In higher tra- 

 cheate animals the 

 tracheal openings 

 have become restrict- 

 ed to certain definite 

 positions by the action 

 of natural selection. 



The sexes are dis- 

 tinct in Peripatus. 

 The males are much smaller and fewer in number than the 



HEAD OF EMBRYO OF PERIPATUS CAPENSIS, 

 SHOWING THE DEVELOPMENT OF THE JAWS. 



