1897- ARE THE ARTHROPODA A NATURAL GROUP? 99 



retained the Apus-like swimming plates under the pygidium. The 

 Crustacea are the only well established group which became 

 arthropods while retaining an aquatic life. 



(4). The Arachnida. The common ancestor of the arachnids 

 was a chaetopod in which the two first pairs of parapodia were 

 thrown forward, two above and two below the mouth, the first pair as 

 seizing limbs, the second pair primarily as sensory, and only 

 secondarily as seizing organs ; the prostomium and labium were 

 squeezed between these to form a beak for sucking the blood of 

 victims crushed out by the above-mentioned limbs. This liquid food- 

 supply, pumped in with force by the oesophagus, as can be shown 

 in detail, modified the whole of the rest of the organisation. The 

 locomotion necessary for catching living prey was highly specialised, 

 four pairs of parapodia behind the two grouped round the mouth 

 developing into long legs, while the posterior region became a great 

 distensible sac with its limbs aborted. 



(5). The Insecta. The primitive insect I believe to have been 

 worm-like, with a first pair of parapodia developed into antennae, and 

 the three following pairs forced forward as jaws to work in an 

 anterior mouth-aperture ; the following pairs were originally but little 

 developed for locomotion, being used rather for clinging to the leaves 

 and stalks on which the insect fed. The herbivorous caterpillar 

 represents, I believe, a true stage, and probably one of the earliest 

 stages, in the development of the Insecta from their chaetopod 

 ancestors. 



(6). The Myriopoda. These have two distinct types of mouth- 

 organs ; if these two different modifications of parapodia cannot be 

 deduced the one from the other, then I should be compelled to regard 

 the Chilopoda and Diplopoda as two distinct derivations from 

 Chaetopoda. There would surely be nothing astonishing in the fact 

 that two (or, including Peripafjis, three) different descendants of the 

 Chaetopoda, on taking to a land life, retained the whole series of 

 parapodia, i.e., behind the mouth-organs, for locomotion. 



(7). Peripatus. I regard this as a feebly developed arthropodan 

 derivative of the Chaetopoda, an arthropod in making, and rightly 

 classed with the rest though in no true sense an ancestral form of any 

 of the foregoing. 



(8). The Pycnogonida. Whether these are to be deduced from 

 any of the above or are again distinct specialisations, I feel unable 

 even to guess ; they remain the most enigmatical of the Arthropoda. 



Two further remarks on these brief notes on the phylogeny of 

 the Arthropoda : — 



(9). I have endeavoured elsewhere to show in detail how the 

 distribution not only of glands but also of the tracheae of the different 

 arthropods may be fully and simply explained by deducing them all 

 (? the coxal glands) from the bristle-sacs of the original chaetopodan 

 parapodia. 



