INTERNAL ANATOMF. 117 



published about the same time as Nalepa's paper, I 

 described the liquid contents of the sacks as highly 

 refractive and oily, floating on the surface of water 

 without mixing ; but I was not sufficiently satisfied 

 as to the function to use any name implying any 

 particular office for the organs ; I therefore called them 

 the " expulsory vesicles as a neutral name. The 

 sacks hardly look glandular in structure, but Nalepa 

 says that the inner side of the sack is clothed with 

 low and small epithelial cells filled with a granular 

 substance. 



RESPIRATION. 



I do not call this section " Organs of respiration " 

 because, as far as is known, the Tyroglyphidse do not 

 possess any special organs by which this process is 

 effected. The family belongs to the atracheate group 

 of the Acarina, in which the blood appears to be 

 aerated simply through the cuticle of the general 

 body-surface without special appliances for the pur- 

 pose. The blood lies in the general body cavity, and 

 is only kept in some degree of movement by the 

 action of the legs and the muscles of the abdomen 

 and cephalothorax ; in active species, however, these 

 are probably sufficient to keep the fluids in almost 

 constant motion ; but not in any definite course : no 

 closed system of organs of circulation is known. The 

 only families of the Acarina in which a heart or any 

 circulatory vessels are known are the Gamasidse and 

 the Ixodidse ; in these an extremely simple one-cham- 

 bered heart with a single pair of ostia and a forward 

 and also a smaller backward aorta exist.* 



It might not have been necessary to mention the 



* Winckler, "Das Herz der Acarinen," in ' Arb. Inst. Wien,' 1886, 

 T. vii, Hft. 1, pp. Ill 117. In Gamasus they had been previously 

 discovered by Kramer, but only very slightly described. No dis- 

 coveries of similar organs in other families have been made up to the 

 present time, although they have been searched for. 



