OCR Text |
Show 12 DR. J. F. GEMMILL ox [May 12, tissue, and (3) two arteries which will be afterwards described, one being a continuation of the ventral aorta, and the other a small artery for the supply of the inner or adjacent pseudobranchs. The two mouth-openings lead into separate buccal cavities, but the oesophagus is single, the septum above mentioned ending opposite the second branchial cartilage. The mandibular apparatus may be described as consisting of a composite arcade underlying the two mouth-openings. The outer portions of this arcade are formed by normal (outer) Meckelian bars {i. e. right bar of right twin head and left bar of left head), while a small mesial portion of the arch is formed by rudimentary adjacent Meckelian bars (PL II. fig. 10, Meek'). These latter bars are united at their proximal ends, and there articulate with the fused adjacent palato-quadrates. Distally, each of the inner bars unites in a symphysis with its corresponding outer Meckelian bar. As the inner bars are exceedingly reduced in size, the two symphyses lie close together in the tissue of the septum separating the two mouth-openings. The hyoid apparatus may also be described as forming a composite arcade, the main part of which consists of the outer arches of the twin heads, while in the middle are interposed the fused remains of the inner arches (fig. 10). These remains consist of (1) a single twisted piece of cartilage articulating, without the intervention of an interhyal, with the fused adjacent hyomandibulars, and representing cerato-hyals ; and (2) two incompletely separated hypohyals, articulating below with two glossohyals which are also incompletely separated. Connected with the outer sides of these glossohyals are the hypo-hyal pieces of the outer arches (PL II. fig. 10, BR). No adjacent elements are interposed in the series of branchial cartilages. The only evidence indicating duplicity is to be found in the second copular piece, i. e. that succeeding the glossohyals. This piece is double anteriorly, but it becomes single opposite the articulation with it of the second branchial cartilage. The succeeding copular pieces are single, but they are a little broader than normal, especially in front. The notochords remain separate as far back as the twentieth somite. For the arrangement of the neural and harmal arch cartilages in the transition region, see above, page 8. There are two sets of brain cavities and masses as far back as the level of the fourth ventricle. The fourth ventricle is single posteriorly, but it bifurcates in front into two canals leading into the separate mid-brains. The posterior part of the medulla and the anterior part of the spinal cord are composite, and show the following characters :-(1) they are much drawn out transversely, and (2) they give origin to small adjacent nerve-roots. These roots in the medulla are extremely rudimentary, and their ultimate distribution could not be traced, but in the spinal cord the nerve-roots in question are better developed and form a regular series of pairs of nerves coming off from the ventral aspect of the cord and distributed to the somites of the median |