366 ARACHN1DA. 



nyssus, Gamasus, Bdella) as are sufficiently transparent are submitted 

 to examination under the microscope, although it is easy to see that 

 the blood or other nutritive juices upon which the creatures live, and 

 with which their bodies are filled, occupies a lobed or symmetrically 

 multifid space, there is no appearance of any canal possessing distinct 

 walls, but the whole seems diffused through lacunae that extend even 

 into the bases of the legs. 



(941.) The Acari, however, possess an anal orifice, through which 

 cxcrementitious matter undoubtedly issues ; nevertheless, on examining 

 this excrementitious substance, it appears rather to present the cha- 

 racters of a secretion, as, for example, in the case of the genus Uropoda, 

 where it becomes consolidated on exposure to the air into a little horny 

 stem, upon which the creature is attached as upon a pedicle. It might 

 therefore, as M. Dujardin observes, be possible to conceive this kind of 

 digestion in a mass acting much in the same way as the glands upon 

 the nutritive juices submitted to their action. 



(942.) In the most simply organized Acaridans, such as Acarus and 

 Sarcoptes, no traces of any respiratory apparatus are discoverable, and 

 respiration seems to be entirely effected by the general surface of the 

 body. In Ixodes, Gamasus, and other Acaridans furnished with forci- 

 pated mandibles, on the contrary, numerous elegantly-ramified tracheae, 

 of which the larger trunks are distinguishable by a spiral filament, re- 

 sembling that exhibited by the tracheae of Insects, are dispersed through 

 the body. These respiratory tracheae communicate externally through 

 the medium of minute stigmata, which in Oribates are situated between 

 the first two pairs of legs. 



(943.) Between these two extremes in the development of the 

 respiratory system of the Acaridans, numerous intermediate grades exist 

 in different genera ; and, in some, M. Dujardin* has pointed out a mixed 

 kind of respiratory process very different from anything as yet observed 

 among articulated animals ; this consists in a system of tracheae ter- 

 minating at a respiratory mouth situated at the base of the mandibles, 

 and serving only for expiration, while inspiration is effected by the 

 general integument and its appendages. 



(944.) To render intelligible this phenomenon, it will be necessary 

 to lay before the reader the description given by M. Dujardin of the re- 

 spiratory (or, rather, expiratory) apparatus as it exists in Trombidium. 

 It is as follows : At the base of the mandibles superiorly is seen an 

 oblong orifice, bounded by two lips, the structure of which is altogether 

 remarkable : it is a perforated eminence (bourrelet reticule a jour), the 

 internal cavity of which communicates with two large tracheal vessels 

 which run parallel to each other from behind forwards to this orifice. 

 Each of the tracheal trunks, at a little distance from the orifice, sud- 

 denly divides itself into a tuft of tubular tracheae, which are without 



* Loc. cit. p. 17. 



