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Show 1895.] HYDBACHNID POUND IN COBNWALL. 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 m e to differ 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 unbrancbed or but little branched ; it is bilateral. As is usual in the Hydrachnidae hitherto examined, what may be considered as the central air-chamber ou 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, tracheaa, &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 mandible works. From the concave side of the lower and hinder portion of the sigmoid piece arise five broad fasciae of muscles (fig. 21, mlm.) 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. W h e n these muscles contract they depress the posterior end of the mandible, and consequently raise its anterior end and clawr, which, as will be noticed in figs. 21, 23, |
