183 
It is searcely necessary to deal here with the Tardigrada and Pycno- 
gonida, as it is evident to any one who has given the matter attention 
that the relations 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 Pentastomum. 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 placed 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 group it has been supposed to connect with the Vermes 
through such forms as Demodex and Phytoptus. 
The hypothesis is an enticing one and has facts which appear to 
support it; but an aberrant form, imperfectly understood, 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 amoment 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-embryonie de- 
velopment indicates that if it isan Acarid it is adegraded 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 
Sarcoptide, 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 Sarcoptide, 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 whichits attacks induce. 
In brief, I can not see in the general resemblance between Pentasto- 
mum, Demodex, and Phytoptus anything more than achance 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 per- 
haps also Pentastomum, sprang. In this case we must look elsewhere 
than to Demodex and Pentastomum for the originals of the typical 
Sarcoptide. <A closely related group of mites, the Tyroglyphide, seems 
to answer most of the requirements; but before giving it further atten- 
