183 



It is scarcely necessary to deal here witli the Tardignula and Pycno- 

 gonida, as it is evident to any one who has given the matter attention 

 that the reUitions of these groups with the Acarina, and even with the 

 Arachnida, are doubtful. 



Several other forms can not be disposed of summarily. Prominent 

 among these is the worm-like Peutastonuim. As you know, its early 

 stages are passed in the lungs and livers of vegetable-feeding mammals, 

 and of reptiles, and in its adult state it occurs in the nasal cavities of 

 Carnivora — an alternation very much like that of Cestodes among par- 

 asitic worms. It was pla(;ed by early naturalists among the Vermes, 

 and its relations with the Arthropoda were first demonstrated by Van 

 Beneden in 1848. Later, Leuckart's investigations confirmed and estab- 

 lished this view of its affinities. Recently it has been placed with the 

 Acarina, which gnmj) it has been supposed to connect with the Vermes 

 through such forms as Dcmodex and Phytoptus. 



The hypothesis is an enticing one and has facts which appear to 

 SUi)port it; but an aberrant form, imperfectly und<'rstood, should not, it 

 seems to me, be allowed to blind us to numerous facts furnished by 

 geology and morphology, indicating the derivation of Arachnida from 

 Crustacea ; and we can not for a moment admit that the group Arachnida 

 as known to us had two independent origins, one through the Crustacea 

 the other through the Vermes. Moreover, Pentastomum shows in its 

 embryology affinities with the Crustacea, and its post-embryonic de- 

 velopment indicates that if it is an Acarid it is a degraded one, its larva 

 being more nearly typical of the Acarina than the adult. 



Demodex is unquestionably a mite, but has every appearance of a de- 

 graded form whose simplicity of structure is to be attributed to disuse 

 consequent on its peculiar habits. It appears to be related with the 

 Sarcoptidse, and might without violence to current ideas on classification 

 be placed in the family. 



Phytoptus, while bearing a general resemblance to Demodex and 

 Pentastomum, is more closely related with the spinning mites than with 

 the Sarcoptidae, and its slenderness of body and the forward position of 

 its legs are evidently developments to favor it in its active life between 

 the scales of buds and in the galls which its attacks induce. 



In brief, I can not see in the general resemblance between Pentasto- 

 mum, Demodex, and Phytoptus anything more than a chance approxi- 

 mation, having no philogenic significance specially pertinent to the 

 subject in hand. 



If I am right, therefore, in holding these forms to be simply extremes 

 of degeneration, then such forms as Sarcoptes are in a sense the lowest 

 in rank, and are probably the source from which Demodex, and x>ct- 

 haps also Pentastomum, sprang. In this case we must look elsewhere 

 than to Demodex and Pentastomum for the originals of the typical 

 Sarcoptidae. A closely related group of mites, the TyroglyphidjB, seems 

 to answer most of the requirements; but before giving it further atten- 



