THE EVIDENCE FROM DEGENERATION. 



355 



animals may acquire lower characters, contrasting with the higher 

 phases to which other members of their class have attained. The 

 mites and ticks have unquestionably originated from the same root- 

 stock as the spiders and scorpions. The development of 

 the two groups proves this much. But whilst the latter 

 animals have advanced to a high complexity of organisa- 

 tion, the mites and ticks have degenerated into parasitic 

 forms or at least exemplify beings which, first attaining a 

 respectable rank in their own series, have certainly not 

 advanced upon that rank. Many of the mites, however, 

 exhibit well-marked degeneration. Only on the hypothesis 

 of sweeping retrogression can we account for the singular 

 and anomalous condition in which a certain harmless 

 mite, named Demodex follicidorum (Fig. 254), spends its 

 existence. This mite inhabits the sacs or follicles of the F >G- 254- 

 human skin at the sides of the nose. It is a minute (magnified), 

 worm-like animal, possessing eight degenerate rudiments 

 of legs, and a thoroughly rudimentary structure in other respects. 

 Here parasitism has denuded the animal of well-nigh every attribute 

 of its Arachnidan character, and has left it in a condition analogous 

 in many respects to sacculina itself. Of the equally curious Lingua- 

 tulina (Fig. 255) inhabiting the "frontal sinuses " or forehead spaces 

 of dogs, wolves, horses, and sheep, 

 the same remark holds good. The 

 body here is thoroughly worm-like 

 in shape (b, c], and a digestive and 

 nervous system are to be enume- 

 rated among the possessions of the 

 organism. But not even the rudi- 

 ments of legs are to be perceived, 

 although the mouth bears certain 

 apologies for the appendages proper 

 to that region in the mite and spider 

 class. Yet the young Linguatulina 

 (a) exactly resembles the early form 

 of the mites. It possesses two pairs 

 of jointed limbs, and certain style- 

 like organs pertaining to the mouth. 

 There is thus the clearest evidence 

 that Linguatulina is a degraded 



animal. It is the degenerate descendant of a free living and ap- 

 parently four-legged or it may be eight-legged ancestor ; and its 

 further history seems to afford a clue to the causes of its retrogression. 

 For the four-legged larva of Linguatulina escape whilst still within the 

 egg from the nose of the dog or sheep host which has harboured their 



A A 2 



FIG. 255. LINGUATULINA. 



