ARACHNOIDEA. 575 
Note.—Genus Myzostoma LEUCcK., containing species that live parasiti- 
cally in Comatule, allied, according to Jou. MUELLER, to the Arctisci, ought 
perhaps to be placed amongst the parasitic crustacea. See below. 
Section II. Holothoraca. Cephalothorax undivided. Organs 
of respiration in most internal, distinct, with two or more stigmata 
in abdomen, serving for the inspiration of air. 
A. Trachex the sole instruments of respiration. 
OrprEr IL. » Acaring. 
Cephalothorax conjoined with abdomen not divided into seg- 
ments. Mouth in many in the form of a rostellum. 
Family III. Acarea. Head terminated anteriorly by labium 
emarginate, or by a single bifid process. Palps connate with labium, 
little distinct. Mandibles chelate. Ocelli none distinct. Feet 
mostly terminated by an adhesive vesicle or acetabulum and claws. 
Compare on this family and the whole order A, DucEs Recherches sur 
V Ordre des Acariens, Ann. des Sc. nat., sec. Série, 1. Zool. Pp. I—4o, pp. 
'44—174; I. pp. 19—63 ; which author we shall principally follow here, 
for want of observations of our own ; also DusJARDIN sur les Acariens, in 
Comptes rend. T. x1x, p- 1158, and Ann. des Sc. nat. 1845, sec. Série, 
Tom. 111. p.§; Niconer Hist. nat. des Acariens qui se trouvent aux environs 
de Paris, Archives du Mus. T. VU. 1855, pp. 381—482. 
Acarus Latr. Tyroglyphus (Ltr. formerly), Gerv. Body 
divided by a transverse furrow between the second and third pairs 
of feet. Four feet approximate at the insertion. 
Sp. Acarus domesticus Du GEER, Acarus siro L. (in part), Herinc, Dr 
Grrr, Mém. vu. Pl. 5, figs. t—11, Lyonrr Recherches, Pl. 14, fig. 15, 
Dumérin Cons. gén. s. 1. Ins. Pl. 52, figs. 4—7 (under the wrong name of 
sarcopte de la gale ;) this small mite, living on old cheese, has in the young 
state, like many Acarida, only six feet. 
2 Myobia V. Hnypen. 
Sp. Sarcoptes musculinus Kocu, HeErnicu-Scumrrer Deutschl. Ins. Heft 138, 
Tab. 13. 
Hypopus Duaus. 
See Dusarpin, Mém. s. des Acariens sans bouche dont on a fait le genre 
Hypopus, et qui sont le premier dge des Gamases, in Ann. des Sc. nat., 3e 
Série, Zool. Tom. x1. 1849, pp. 245, 250. r 
