160 HINTS ON THE ANCESTRY OF INSECTS. 



for the transmutation of any worm with which we are at present 

 acquainted into a form like the Leptus, with its mandibulated 

 mouth and jointed legs, seems at first well nigh impossible. 

 We have the faintest possible indication in the structure of 

 some mites, and of the Tardigrades and Pentastoma, where 

 there is a striking recurrence, as we may term it, to a worm-like 

 form, readily noticed by every observer, whatever his opinion 

 may be on the developmental theory. In the Demodex we see 

 a tendency of the mite to assume under peculiar circumstances 

 un elongated, worin-like form. The mouth-parts are aborted 

 (thonjrh from what we know of the embryology of other mites, 



they probably 

 are indicated 

 early in embry- 

 onic life), while 

 the eight legs a re 

 not jointed, and 

 form simple tu- 

 bercles. In the 

 Tardigrades, a 

 long step lower, 

 we have un- 

 Mointed fleshy 

 I legs armed with 

 from two to four 

 | claws, but the 

 mouth-parts are 

 essentially mite 

 in character. A 



191. NauplhiB. decided worm 



feature is the fact that they are hermaphrodites, each individual 

 having ovaries and spermaries, as is the case with many worms. 

 When we come to the singular creatures of which Fentastoma 

 and Linguatula are the type, we have the most striking approx- 

 imation to the worms in external form, but these are induced 

 evidently by their parasitic mode of life. They lose the rudi- 

 mentary jointed limbs which some (Linguatula especially) have 

 well marked in the embryo, and from being oval, rudely mite- 

 like in form, they elongate, and only the claws or simple curved 

 hooks, like those of young tape worms, remain to indicate the 

 original presence of true jointed legs. 



