KNOWLEDGE OF THE HYDRACHNIDAE. 277 



structures as dev^eloped in Feripatus than with those of any 

 other arthropod. In certain histological details they neverthe- 

 less differ conspicuously from the glandular structures, mucous 

 or otherwise, hitherto recorded. Notwithstanding, in fact, that 

 these structures have been carefully examined under the highest 

 magnifying powers both fresh and after treatment with osmic 

 acid and many other reagents, it has been found impossible to 

 detect the presence of any cellular elements. The w^alls of the 

 tubuli themselves, while rapidly absorbing coloured media, 

 exhibit no differentiation whatever ; a circumstance that would 

 appear to warrant their interpretation as chitinous structures 

 which, dipping down into the cellular elements of the somatic 

 -cavity, derive thence, by endosmosis, their characteristic muci- 

 laginous existence. It is worthy of note that in connection 

 with certain media, such as pure glycerine, the contents of 

 these tubuli become highly vacuolate, while their outer walls 

 become variously dilated and distorted, or may even coalesce 

 mth those of neighbouring tubuli and apparently thus form 

 one homoEjeneous mass. 



Reproductory System. 



The sexes of the Hydrachnidae, as in the majority of the 

 Acarina, are distinct, the ova or seminal elements, as the case may 

 be, being developed in reproductive sacculi that open upon the 

 usually ventrally and mesially located genital aperture. Accord- 

 ing to Claparede, the ripening ova become detached and lie loosely 

 within the somatic cavity. This interpretation has, however, 

 been called in question by Kramer, and is opposed to the results 

 obtained by the author of this paper by means of sections made 

 in connection with a variety of species, a like relationship of 

 these parts being also found to obtain in association with the 

 testes and their products. Reference has already been made 

 to the specially modified chitinous indurations of the cuticular 

 surface that bound each side or are developed within the neigh- 

 bourhood of the genital aperture. These structures, w^hich vary 

 to an almost indefinite extent, are found not only to afford 

 reliable data for specific discrimination, but both alone or in 

 correlation with other structural details are found to yield a 



