OCR Text |
Show 638 MR. W. G. RIPEWOOP ON THE [Nov. 6, hyoidean nerve, instead of lying between the hyomandibular and the cranium, penetrates the cranium itself, and the hyomandibular is separated by a wide gap from the tubercle to which the hyosuspensorial ligament is attached. It is not impossible that the bar of cartilage behind the nerve-aperture is the symplectic process, which has fused above with the cranium and below with the tubercle (which latter in the specimen represented in fig. 3, e, is practically obsolete); but it is more reasonable to suppose that the nerve has been enclosed by the posterior union of the edges of the notch in the cranial cartilage in which it originally ran, parallel cases of the enclosure of nerves and blood-vessels being not uncommon. Little doubt can be entertained of the fact that the whole branchial and hyoid system has become reduced in Ceratodus in relation with the partial adoption of pulmonary respiration; and the hyomandibular thus furnishes another and a very interesting example of individual variation in vestigial structures. This reduction of the branchial skeleton is even more pronounced in Protopterus, where the hyomandibular is absent and where only the second and third branchial arches have epibranchial elements, the other four consisting of slender ceratobranchials only. The hyoid gill-filaments in Ceratodus are set on the ventro-internal edge of the ceratohyal, and the series extends backwards and upwards, internal to the hyosuspensorial ligament, but stops short for about a quarter of an inch in the region of the hyomandibular and is continued again in a line running back, parallel to the long axis of the body, to meet the upper end of the series of gill-filaments of the first branchial arch. Thus not only is the continuity of the gill broken in the region of the hyomandibular, but the direction of the series of filaments becomes changed. The gill-rakers, on the other hand, form a continuous series running along the upper and inner edge of the ceratohyal, curving forwards and upwards in front of the hyomandibular, and terminating at the upper extremity of the first branchial cleft. While the series of hyoid gill-rakers and gill-filaments are close together along the inner surface of the ceratohyal, they become widely divergent above, the upper end of the first branchial arch being situated some little distance behind the pharyngeal opening of the cleft. The upper gill-filaments are not supported by any part of the hyoid skeleton, but simply project from the mucous membrane and readily come away when the latter is stripped off, and the lower filaments, although supported by the ceratohyal, are but very feebly attached to it and leave no marks on removal, the gill-filaments of Ceratodus not being supported by gill-rays. As before mentioned, the hyoid arch of Ceratodus is less reduced than in the other living Dipnoi. In Protopterus there is a pedicle for the attachment of the hyosuspensorial ligament, situated much as in Ceratodus, and to the inner side of this short ligament lies a much longer and whiter |