Glass 4. Arachnida. Order 3. Acarina. 



285 



2. Itch-mites (SarcopHdie) are microscopic, blind, without traclie», and 

 generally witli suckers at the tips of the feet; they live as stationary parasites 

 on Mammals and Birds, and feed either upon the skin or upon exuded lymph. 

 There is a marked sexual dimoi-phism, and it is interesting to note that copula- 

 tion occui's before the females have reached the adult form, and whilst the ovary 

 is still entirely undeveloped. Here belongs the human Itch-mite (Sarcoptes 

 scabiei) which buiTOws in the epidermis ; the female has suckers on the two 

 anterior paii-s of legs, the small males also have them on the fourth pair. 

 Various nearer or more distant allies live in and on the skin of other Mammalia 

 and Birds, causing mange. Peculiar microscopic Mites of elongate form and with 

 short legs, destitute of suckers {DemodexfolUculorum) occur in the hair follicles 

 of the human nose ; they are quite harmless, but a variety of the same species 

 which is parasitic upon Dogs, causes a very bad skin disease in this animal. 



3. G-all-mites (Phytoptus) are microscopic, with elongate bodies, and 

 are easily distinguished from other Mites by the possession of only the two 

 anterior pairs of legs. They suck the sap of plants and thus cause vaa-ious 

 abnormal growths of leaves and buds of many, 

 ■especially woody, plants. h, -.., 



Appendix to the Arachnida. 



The three groups following are very peculiar, 

 and their systematic position is not at all certain ; 

 they are generally, however, regarded as belonging 

 to the Arachnida, and this is probably true for 

 the first two. They are, however, so very aberrant 

 in all respects that they are best considered in an 

 appendix. 



The Fentastoma (Pentastomum) live para- 

 siticaUy in various Vertebrata ; they are animals 

 of very considerable size, and at first sight are 

 very like short-jointed Tapeworms. The body is 

 elongate, usually flattened, and separated by 

 fun-ows into a large number of short segments ; 

 the segmentation is, however, only external and 

 does not affect the internal structure ; the limbs 

 are only represented by two pairs of chitinous 

 hooks which are situated anteriorly not far from 

 the mouth. The alimentary canal is a straight 

 tube, the anus posterior. The male genital 

 aperture lies far forward on the ventral surface, 

 the female pore close to the anus ; the genitalia 

 recall those of the Arachnida. The central 

 nervous system is reduced to a ventral ganglion 

 below the pharynx and a ring, arising from it 

 to run round this. Sensory, respiratory, and 

 circulatory organs are absent. Peniastomum 

 txnio'ides, when sexually mature, inhabits the 

 nares and frontal sinuses of the Dog and the 

 Wolf (female 8 c/m. and upwards, male 2 o/m. 

 long). The ova escape with the mucus of the 

 nose, and each contains a young Pentastomum, 



possessing two pairs of small hooked legs. If these ova are ingested by a Hare 

 or Rabbit, the egg sheU is dissolved in the stomach, and the small animal makes 

 its way into the liver, where it grows considerably, but does not become sexually 

 mature. Immature Pentastoma are also now and then found in other Mammals 



Kg. 232. Female of Pentn- 

 stomum taenioides. d gut, h 

 hooks, oe oesophagus, ov ovary, 

 rs seminal vesicle, va oviduct. — 

 After Leuckart. 



