276 ARTHROPOD A [CH. 



indeed were formerly classed with them. Like Arachnida, none of 

 their limbs are converted into jaws, indeed none of them have any 

 process which could be called a gnathobase and the first pair, the 

 chelophores, have the form of a little pair of pincers like the 

 chelicerae of Arachnida but are three-jointed not two-jointed like 

 chelicerae. There are also four pairs of long many-jointed legs. 

 But in other points they differ widely from Arachnida. None of the 

 legs are situated in front of the mouth ; this opening is found at the 

 end of a snout-like outgrowth of the body called the proboscis. 

 Further, when all the legs are developed as is the case in the genus 

 Nymphon, it is found that there are three pairs of reduced legs in 

 front of the first walking leg so that the walking legs cannot exactly 

 correspond in Arachnida and Pantopoda. The third pair of reduced 

 legs 'are always developed in the male even when absent in the 

 female, and when the female lays the eggs the males attach them to 

 these "ovigerous legs" and carry them about till they are hatched. 



There is no abdomen and no tracheae, and the genital organs in 

 many cases have several pairs of ducts opening at the bases of the 

 walking legs. This last feature definitely separates Pantopoda from 

 Arachnida and establishes their right to be regarded as an indepen- 

 dent sub-division of Arthropoda. 



The TAEDIGEADA are minute Arthropoda which live amongst moss 

 and heather ; they are of microscopic dimensions. Their cuticle is 

 so thin that we cannot distinguish sclerite and arthrodial membrane. 

 In this they resemble Peripatus and they also resemble Peripatus 

 in the form of their appendages of which they have only four pairs. 

 These appendages are short stumpy legs each ending in two claws. 

 Perhaps another resemblance to Peripatus may be seen in the fact 

 that there is a globular buccal cavity into which two stilet-like 

 lancets project, by which the Tardigrada pierce the cells of the 

 plants on whose juices they live. It will be remembered that in 

 Peripatus the single pair of jaws have the form of blades inside a 

 buccal cavity surrounded by a lip. The genital organ opens into 

 the rectum behind and so do two Malpighian tubes. This last 

 feature separates them from Peripatus, and if we are not prepared 

 to regard the Tardigrada as minute degraded relatives of that 

 primitive form we must accord them the dignity of an independent 

 class of Arthropoda. 



