KNOWLEDGE OF THE HYDRACHNIDAE. 271 



disposition of this coloured excretory structure in the roain holds- 

 good for each species, and hence furnishes a valuable accessory 

 for the purposes of specific diagnosis. 



Circulatory and Respiratory Systems 



No heart or other vascular apparatus has so far been dis- 

 covered in the Hydrachnidae or Acarina generally, nor to 

 compensate for such absence is the vital fluid propelled, as in 

 many lower arthropods, by the muscular contractions of the walls 

 of the perivisceral cavity. In place of this, as first pointed out b}'" 

 Claparede,* the amoebiform blood corpuscles are endowed with 

 extraordinary vitality and creep freely about within the liquid 

 plasma contained within the lacunae or interspaces of the limbs 

 and body cavity. 



Concerning the respiratory system of the Hydrachnidae, it 

 has fallen to my lot to place on record an account of certain 

 structural and physiological details that have seemingly escaped 

 the notice of all previous investigators. Since the time of 

 Duges, the possession by the members of this group of an 

 extensively developed tracheal system has been generally 

 recognised, different authorities, how^ever, being at variance with 

 respect to the manner in which these tracheae are brought into 

 relation with the external surface of the body. By many of 

 these it was supposed that either all or a certain number of the 

 minute circular apertures or so-called stigmata, distributed 

 symmetrically in varying positions upon the dorsal and ventral 

 surfaces of the body, were the direct openings of these tracheae 

 as obtains among the ticks and other terrestrial Acari and 

 higher Arachnida. As, however, first demonstrated by Claparede t 

 in the case of Atax crassvpes and A. yps'doj^horus, these stig- 

 mata are the excretory orifices of special integumentary gland; 

 and an analogous function and import, so far as is at present 

 known, must be connected with these structures in all other 

 Hydrachnidae. 



-* ' Studien an Acariden," Zeitsch. Whs. Zooh, Bd. xviii., Hft. iv., 1868. 

 t Ihlcl., p. 479. 



*^^ 



