1895.] HTDRACHNID FOTTSTD IK CX)KNWALL. 203 



genital nerve are very numerous, and I am not prepared to deny 

 that some of them may serve other organs not belonging to the 

 genital system. 



Besides these paired nerves there is a fine azygous recurrent 

 nerve in the median line (figs. 20, 23, nr.) running below that 

 portion of the oesophagus which lies between the brain and the 

 ventriculus, and innervating the latter organ, or at all events the 

 ventral surface of it. 



The histology of the great nerve-centre does not appear to me 

 to difiier sufficiently from what has been described to need remark ; 

 the principal point which attracts attention is the great thickness 

 of the structureless neurilemma, below which is a single layer of 

 the usual small round cortical cells coating the fibrous material of 

 the brain, but much less conspicuous than is generally the case in 

 Acarina. 



The Respiratory Organs (Plate VIII. fig. 21 ; Plate IX. fig. 23). 



These do not vary very greatly in the present species from 

 what has been before described ; there are, however, some points 

 worth recording. 



The system is strictly tracheate, and the tracheae are very 

 numerous, very fine, and mostly unbranehed or but little branched ; 

 it is bilateral. As is usual in the Hydrachnidse hitherto examined, 

 what may be considered as the central air-chamber on each side 

 of the body is a somewhat S-shaped piece of chitin which I will 

 call the "sigmoid piece" (figs. 21, 23, sp.); it is not, however, 

 truly S-shaped in the present species, the lower half of the S 

 being much more developed and curved than the upper. This 

 piece of chitin is flattened laterally, and the two pieces are very 

 near each other and consequently very near the median line of the 

 body, one being on each side of the line, each is nearly at right 

 angles to the mandible on its own side ; the chitinous tube of the 

 mandible is sharply cut away on its inner side about two-thirds of 

 its length from the anterior end, leaving an oval hollow at the 

 inner posterior third of the mandible into which muscles, tracheae, 

 &c. pass. The chitin of the mandible forms a concavity which 

 rests upon the head of the sigmoid piece, which thus forms a 

 fulcrum upon which the maudible works. From the concave side 

 of the lower and hinder portion of the sigmoid piece arise five 

 broad fasciae of muscles (fig. 21, nilm.) arranged in a fan-shape ; 

 each fascia is attached to the sigmoid piece by numerous very 

 short tendons similar to those attaching the pharyngeal muscles to 

 the roof of the pharynx, but shorter. The five fasciae converge 

 and are inserted into the inner edge of the hind (cut away) portion 

 of the mandible, each fascia being attached by more than one 

 tendon ; these tendons are less numerous, but slightly longer, 

 than those at the sigmoid end. When these muscles contract they 

 depress the posterior end of the mandible, and consequently raise 

 its anterior end and claw, which, as will be noticed in figs. 21, 23, 



