B; a, genital orifice;
b, anus; c, united basal segments of the second pair of appendages;
d, basal segment of the 6th prosomatic appendage of the right side.
The rest of the appendage, as also of app. Ill, IV and V, has been
cut away.]

Sub-order e. _Astigmata._--Degenerate, mostly parasitic forms
approaching the Prostigmata in the development of integumental
sclerites and the softness of the skin, but with the respiratory
system absent.

Families--Tyroglyphidae (_Tyroglyphus, Rhizoglyphus_).
Sarcoptidae (_Sarcoptes, Analges_).

Sub-order f. _Vermiformia._--Degenerate atracheate parasitic forms
with the body produced posteriorly into an annulated caudal
prolongation, and the 3rd, 4th, 5th and 6th pairs of appendages short
and only three-jointed.

Family--Demodicidae (_Demodex_).

Sub-order g. _Tetrapoda._--Degenerate atracheate gall-mites in which
the body is produced posteriorly and annulated, as in _Demodex_, but
in which the appendages of the 3rd and 4th pairs are long and normally
segmented and those of the 5th and 6th pairs entirely absent.

Family--Eriophyidae (_Eriophyes, Phyllocoptes_).

_Remarks on the Rhynchostomi._--The Acari include a number of forms
which are of importance and special interest on account of their
parasitic habits. The ticks (_Ixodes_) are not only injurious as
blood-suckers, but are now credited with carrying the germs of Texas
cattle-fever, just as mosquitoes carry those of malaria. The
itch-insect (_Sarcoptes scabiei_) is a well-known human parasite, so
minute that it was not discovered until the end of the 18th century,
and "the itch" was treated medicinally as a rash. The female burrows
in the epidermis much as the female trap-door spider burrows in turf
in order to make a nest in which to rear her young. The male does not
burrow, but wanders freely on the surface of the skin. _Demodex
folliculorum_ is also a common parasite of the sebaceous glands of
the skin of the face in man, and is frequent in the skin of the dog.
Many Acari are parasitic on marine and freshwater molluscs, and others
are found on the feathers of birds and the hair of mammals. Others
have a special faculty of consuming dry, powdery vegetable and animal
refuse, and are liable to multiply in manufactured products of this
nature, such as mouldy cheese. A species of Acarus is recorded as
infesting a