KNOWLEDGE OF THE HYDRACHNIDAE. 263 



and thorax or cephalotliorax of the higher Arachnida being 

 indistinguishably fused together. The nearest approach to a 

 deviation from this simple type of structure is found in the 

 males of many species of the genus Arrheniirus, the posterior 

 extremity of the body in many of these forms being greatly pro- 

 longed and not infrequently separated by a more or less incisive 

 constriction from the anterior region. Even in these more 

 exceptional forms there is, however, no distinct line of demarca- 

 tion or segmentation. 



Proceeding to a brief enumeration of the morphological 

 characters of the Hydrachnidae such being common also 

 to the majority of the Acaridae, or indeed of the entire class 

 Arachnida it is found that they possess but six pairs of jointed 

 appendages, the two anterior of these, the mandibles and palpi, 

 pertaining to the oral and the four succeeding pairs to the 

 ambulatory or locomotive systems. In addition to these we 

 have the eyes and a pair of antenniform setae developed in 

 advance of the mouth, and behind the locomotive limbs what 

 seem very like the modified basal joints or epimera of the 

 appendages belonging to a ninth body segment, the entire series 

 following each other in the subjoined consecutive order : 



1. Eyes. 



? 2. Antennary bristles 



3. Mandibles. 



4. Maxillary palpi. 



5. 1st 



6. 2nd 



8. 4th 



^ - Epimera and attached pairs of locomotive limbs. 

 3rd 



? 9. Frontal plates or epimera. 



As will be observed, the figures indicative of the suggested 

 homologues of the second and ninth pair of appendages in the 

 foregoing scheme are aflixed only tentatively. It has long since been 

 generally conceded that the Arachnida as a class are deficient 

 in one of the two pairs of antennary elements possessed by such 

 typical arthropoda as the Crustacea, the pair that is retained 

 indeed not existing under the form of ordinary antennae, but 

 being metamorphosed in such a manner as to constitute the so- 

 called chelicerae or mandibular elements of the ordinary spiders. 



