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Show 1895.] HYDBACHN1D POUND IN COBHWALL. 199 The Nervous System (Plate VIII. fig. 20; Plate IX. figs. 23, 27). I do not know that upon this part of the anatomy I have many observations to describe relative to the present species which differ in very important matters from what has been before observed by other acarologists in various species ; but still I think that there are some new points of considerable interest to be detailed ; and moreover, as former anatomists have not been altogether agreed as to the distribution of the nerves, fresh investigations may be useful, although made upon different species, or even families. The great central nervous mass in all Acarina which have been investigated is the so-called brain (br.); which is penetrated by the oesophagus, that organ passing right through it, generally in a more or less oblique direction, and being accompanied by tracheae in the present species. Although the whole of this brain is one mass, yet its formation from a supra-cesophageal and a sub-cesophageal ganglion is usually fairly apparent; the latter frequently extending considerably further backward than the former. In the present species the distinction between the upper and lower ganglia is practically lost; the whole forms one almost, but not quite, globular mass (figs. 20, 23, br.) which, in the male, has a diameter of about *13 m m . in a dorso-ventral, and of about -1 m m . in an antero-posterior direction ; it lies considerably nearer to the ventral than the dorsal surface, indeed its lower edge nearly reaches the ventral cuticle. This brain is situated about as far back as the second pair of legs ; it lies below the salivary glands, and in front of the genital aperture, and is invested by a most distinct neurilemma, which is separated from the nervous substance by endosmosis if the organ be soaked in water. The oesophagus (figs. 20, 23, ce.) penetrates the brain in a slightly oblique direction, running backward and a little upward. From just above the oesophagus there starts from the lower part of the supra-cesophageal portion of the mass a fine, central, azygous nerve (nph.), which runs almost parallel to, but a little above, the oesophagus for the whole length of that organ; it then splits up into a large number of separate twigs, one of which runs to each muscle of the sucking-pharynx. About this nerve I do not feel any doubt whatever; I have it in several preparations, and in one fortunate sagittal section of the creature I have the whole length of the nerve from the point where it issues from the brain to its ultimate distribution to the pharyngeal muscles. A precisely similar nerve has been figured by Henkin (op. cit. fig. 7) as existing in Trombidium fuliginosum. Winkler1 has drawn a similarly placed nerve in Gamasus, but I imagine that he considers that it goes to the lingula, as he calls it the " Zungennerv"; he however identifies it with Henkin's. Schaub does not mention any such median nerve as going to the 1 " Anatomie der Gamasiden," Arbeit, d. zool. Ins, Wien, vii. p. 336, taf. iii, fig. 8 (1888). |
