266 W. SAVILLE-KEXT, CONTRIBUTIONS TO A 



organs are generally recognisable in the Hydrachnidae. In one 

 species of Arrheiiuras I have nevertheless succeeded in discov^er- 

 ing a structure which bears a most remarkable resemblance to an 

 auditory capsule, with enclosed otoliths imbedded within the second 

 joint, from the base of the first pair of ambulatory limbs. Up to 

 the present time, and notwithstanding the well-known circumstance 

 that many of the higher Arachnida possess the sense of hearing 

 in a marked degree, the precise location, or indeed the character, 

 of the auditory organs does not appear, so far as I can gather, 

 to have been recorded in connection with any representative of 

 this class of the Arthropoda. The seemingly anomalous position 

 of these auditory structures, if such they be, in Arrhenurus is 

 reconcilable with the circumstance that while in the majority of 

 the podopthalmous Crustacea homologous organs are imbedded 

 within the basal joints of the antennules, in other members of 

 the same group, e.g. Mysis, they may be found similarly immured 

 within the endopodites of the last abdominal somite. In all the 

 Hydrachnidae, with but one or two exceptions leading a free- 

 swimming existence, it is found that their oral armature conforms 

 very closely with that of the typical Arachnida, and especially 

 with that of the true spiders or Araneida, and is neither meta- 

 morphosed nor disguised by atrophy, as so generally obtains 

 among the more numerically abundant parasitic Acari. The 

 more anterior or mandibular element in this oral system consists 

 of a thicker basal joint surmounted by a curved ungulate or 

 ensiform very sharply pointed terminal prolongation, the two 

 combined composing an appendage that bears a considerable 

 resemblance to the homologous organs or so-called chelicerae 

 of the typical spiders. The function that they perform is 

 likewise substantially identical, it being with these weapons 

 that the captured prey, consisting for the most part of living 

 Entomostraca, or minute water-insects, is perforated and held in 

 close proximity to the suctorial mouth. As is well known, in 

 the case of the true spiders a poison-gland communicating by 

 a fine duct opens upon the apical extremity of the chelicerae. 

 Although no such lethal structure has so far been recorded in 

 connection with the Hydrachnidae, I have recently succeeded in 

 detecting what I believe to be an organ of corresponding value 

 in several species, notably in Limnesia histrionica and also 

 in various terrestrial Acari. The very circumstance of the 



