372 Introduction to Animal Morphology. 



dcferens may have accessory glands (three pair in Argas). The 

 young are generally hexapod. Eight families are included : 

 i. Pentastomidae nearly colorless, worm-like endoparasites- 

 from the internal cavity of Vertebrates, with a long annulated 

 body ; beginning life as an ovate embryo with two pairs of 

 short, two-jointed, two-clawed feet ; mouth with two hook-like 

 antennaryjaws ; this larva becomes pyriform, a tail forming, 

 with terminal bristles, and after a few moults the hooks are 

 lost, and are followed by others which form an anterior series, 

 two on each side of the mouth. The worm-like adult has a 

 simple digestive tube, with villiferous ridges internally and a 

 terminal anus. The mouth has a chitinous ring and a pair of 

 small salivary glands. The testis is single, sub-gastric at 

 the anterior one-third of the body ; the two vasa deferentia 

 are united at their origin, each passes forwards convoluted, 

 and ends in a long penis, coiled when at rest, and included in 

 a sac projecting near the anterior end. The ovary is long, 

 simple, over the stomach, with its oviducts opening anteriorly 

 into a sac, beside the pharyngeal ganglion, which communi- 

 cates with a pair of large oval lateral receptacula seminis ; 

 from this sac one or two vaginae pass backwards, opening 

 preanally. There are stigma-like pits, but no tracheae. There 

 is one family and genus. P. toenioidcs inhabit the nostrils, 

 &c., of dogs; P. imperatoris and proboscideum, the lungs of 

 the boa, &c. They are the only Arthropods with a continu- 

 ous clothing of longitudinal muscular fibres, which, like the 

 muscles of the intestine, are striped. There is an cesophageal 

 nerve ring. The larvae of some are encysted in the liver of 

 the hare, &c. P. constrictum has been found in the human 

 liver. 2. Acaridae, Mites minute, oval or elongate, with soft 

 body and chitinous supports for the short legs, which have 

 each two claws, and often an adhesive disk ; mouth mandibu- 

 late or suctorial ; ocelli and tracheae none ; often parasitic. 

 Demodex, from the human sebaceous follicles, has a worm- 

 like, unringed abdomen. Sarcoptes, the itch insect, has an 

 oval, undivided body, and the feet all (in the males), or the 

 foremost two pair (females), with suckers ; the back bears 

 conical processes, and the male has three testes and a spicu- 

 lum. Acarus, the cheese and flour mites, have scissors-like 



