RESPIRATION OF ACAKIDANS. 367 



any internal spiral filament, and which are distributed without any 

 ramifications throughout all parts of the body. On observing a living 

 Trombidium, it is seen frequently to agitate its mandibles as though to 

 produce some movement of the air contained in the respiratory apparatus, 

 and if at the same time a little water be placed upon the respiratory 

 orifice, it is sometimes seen to become inflated with little bubbles of air. 



(945.) On dissecting a Trombidium, there is seen beneath the in- 

 tegument, which is covered with plumose hairs, an elegant network, 

 made of a diaphanous substance of homogeneous appearance, which 

 appears to be in relation with the plumose appendages of the integu- 

 ment, and in concert with them serves to absorb the gaseous elements 

 that are subsequently emitted externally through the tracheal orifices. 



(946.) In Hydrachna and the aquatic Acaridans, the expiratory 

 system is similar to that which exists in Trombidium, only that, instead 

 of plumose hairs upon the surface of the body, there are stomata some- 

 thing resembling those of plants that is to say, apertures covered over 

 with a very delicate membrane, beneath which is a subcutaneous net- 

 work, such as exists in the terrestrial species. 



(947.) The nervous system in Trombidium, and probably of the 

 other Acari, presents a very remarkable arrangement, consisting en- 

 tirely of a single large globular ganglion, from which nervous filaments 

 are given off, both before and behind. The researches both of Trevi- 

 ranus and M. Dujardin deny the existence of anything like a supra- 

 oasophageal ganglion or nervous collar around the oesophagus. 



(948.) The eyes of the Acaridans are generally four in number, 

 sessile, and approximated together in pairs upon the dorsal surface of 

 the cephalothorax. In some cases, however, the eye is solitary, and 

 composed of eight or ten minute facets. 



(949.) Trombidium is the only Acaridan in which either M. Dujar- 

 din or Treviranus could discover the presence of a tubular two -branched 

 ovarium ; generally speaking, throughout this order of Arachnidans, the 

 ova are produced in the substance of the general tissue of the body, 

 without the presence of any ovarian apparatus with distinct parietes 

 being apparent. The genus Oribates produces living embryos covered 

 with a soft and wrinkled integument, which, as its development ad- 

 vances, becomes hard and crustaceous ; in these Acari, therefore, in 

 order to enable them to bring forth their young, it is necessary that the 

 orifice of the vulva shall be of extraordinary dimensions ; and accordingly 

 it is found to be, in species thus constituted, a large oval orifice, occupy- 

 ing one -third or one -fourth of the entire length of the body, the opening 

 being closed by two valves. In front of this large orifice, which is placed 

 posteriorly, is another round opening, likewise terminated by valves, 

 which gives issue to a long membranous tube, folded longitudinally and 

 furnished with retractor muscles. It would appear possible, therefore, 

 that this is a penis, and that Oribates is hermaphrodite ; for, seeing that 



