KNOWLEDGE OF THE IIYDEACHXIDAE. 269* 



structures, which may be conveniently distinguished by the title- 

 of the genital plates, are not met with among the higher Arach- 

 nida or true spiders, a closely analogous element occurs in those 

 singular organs, the so-called crab-like structures or "pectines," 

 developed one on either side immediately to the rear of the 

 genital aperture in the scorpions. The precise import of these- 

 *' pectines " in the scorpion does not, so far, appear to have 

 been determined, though it is very likely they are subservient 

 in some way to the phenomena of copulation. These appar- 

 ently closely analogous structures, the genital plates of the 

 Hydrachnidae, are unquestionably associated with a coincident 

 function, and are indeed in many cases provided with acetabular 

 surfaces that assist in the close union of the genital areas during 

 the pairing of the sexes. As will be presently shown, the 

 contour and pattern of these genital plates, or, as here suggested, 

 basal elements of the ninth pair of somital appendages, afford 

 valuable aid both in forming the generic diagnoses of the group 

 and in discriminating between closely allied species. 



Alimentary System. 



The alimentary tract in the Hydrachnidae, as with the higher 

 members of' the same class, is subdivisible into three distinct 

 regions of a fore-gut, mid-gut and hind-gut. The fore-gut or 

 pharynx and oesophagus, leading from the mouth to the stomach,, 

 is straight and of but short length. The two glands apparently 

 partaking of the same nature as the poison glands of the true- 

 spiders, and doubtless the modified homologaes of ordinary 

 salivary glands, previously referred to, may be regarded as 

 appendages of the anterior or fore-gut. The mid-gut or stomach, 

 as in the typical spiders and other Arachnida, is conspicuous for 

 its development of caecal diverticula. In a large number of forms 

 these caeca are found to follow roughly, as viewed sternally, a 

 quincuncial plan of disposition. The single median element in such 

 quincunx is stationed in advance of the central point, while the 

 two postero-lateral caeca are of more considerable length than 

 those pertaining to the central or antero-lateral systems. In an 

 almost equally considerable series the antero- and postero-lateral 

 caeca coalesce with one another on either side in such a mannei 



