INTERNAL ANATOMY. 77 



The above, however, is not the only manner in which 

 the urinary white matter, if it be urinary, finds its 

 way out of the cells of the ventriculus ; it probably 

 passes out in solution in considerable quantities into 

 the body cavity. Bernard also thinks that crystals 

 and concretions pierce the wall of the canal and pass 

 out; he thinks that minute cells not distinguishable 

 from blood-corpuscles are formed in the ventricular 

 cells, and that they really are blood-corpuscles, which 

 finally pass through the walls of the canal, carrying 

 the excretory crystals with them ; this, however, seems 

 a somewhat bold suggestion. It appears, however, 

 certain that in some manner or other the urinary 

 matter, either in a fluid or solid form, gets into the 

 body cavity in large quantities. In Acari, such as 

 Gamasidae, Trombididae, Hydrachnidge, etc., which have 

 large and actively functional Malpighian vessels,* 

 these organs eliminate the urinary matter, and from 

 them it is discharged, either directly or indirectly, by 

 the anus. In creatures like the Tyroglyphidae, how- 

 ever, in which the Malpighian vessels are comparatively 

 small and sometimes apparently absent altogether, 

 the urinary matter (or some part of it) forms crystals 

 or concretions which seem to remain permanently in 

 the body cavity, which in old specimens of such species 

 as Histiosfoma rostro-serratum and Hericia Robini is 

 absolutely crowded with these objects ; it is perhaps 

 worthy of remark that these two species which show 

 the accumulation of the concretions in such remarkable 

 quantities are both waders, which practically pass their 

 whole existence almost immersed in the fluids upon 

 which they subsist. 



* It must not be understood that the Malpighian vessels of these 

 three families are homologous; their functions are analogous, but 

 while the organs in the Gamasidee are clearly homologous with those 

 of the Tyroglyphidae, the great single so-called Malpighian vessel 

 found in the Trombididai and Hydrachnida3 appears probably to be a 

 modification of the principal line of the hind gut. See Michael, "A 

 Study of the Internal Anatomy of Thyas petrophilus," 1895, in ' P. 

 Zool. Soc..' London, pp. 185188. 



