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Show 1903.] MONSTROSITIES IN FISHES. 13 muscular mass which lies ventral to the notochords (compare page 9). In this monstrosity, as in the one previously described, the anterior part of the spinal cord, though it lies nearer to the place of union of twin bodies, shows greater structural duplicity than does the medulla oblongata. All the outer cranial nerves belonging to the twin heads are normal and need no further mention. Of the inner or adjacent nerves, the 1st, 2nd, 3rd, and 4th are normal; the 5th and its ganglia are reduced in size; the 7th and 8th with their ganglia are very rudimentary; while only a remnant of the adjacent glosso-pharyngeal and vagus remains. I was unable to follow out the 6th pair, but the presence of well-developed external rectus muscles makes it probable that these nerves were present. As has already been mentioned, the inner or adjacent auditory capsules are much reduced in size, their auditory sacs being completely united and forming a single labyrinth, symmetrical in shape, compressed from side to side, and receiving the two adjacent auditory nerves. The arrangement of the sensory epithelium inside the various parts of this labyrinth is also bilaterally symmetrical. A reconstruction drawing of this labyrinth is given in PL IV. fig. 27. Saccule, utricle, and anterior and posterior semicircular canals are all represented, but there is no trace of a horizontal semicircular canal. The heart is normal and gives origin to a single ventral aorta, which for a short distance upwards has a double cavity, owing to the presence of a median antero-posterior septum, which, however, disappears further forwards. The gill-arteries on either side are normal, but in addition to them the ventral aorta gives rise to several small irregular branches which ramify in spongy tissue surrounding the ventral ends of the branchial cartilages, and which may be taken to represent a very rudimentary set of adjacent gill-arteries. But the most striking feature of the ventral aorta is that, instead of ending in the first gill-arteries, it is continued forwards and arches dorsally in the tissue of the septum between the two mouth-openings. Passing through the space between the adjacent glossohyals and the succeeding copular piece, it comes to lie behind the small cartilage which represents adjacent ceratohyals. Then, reaching the base of the skull, it bends backwards, and divides into two equal branches which join the upper aortic roots on either side (PL II. fig. 13). In the first part of its course, this continuation of the ventral aorta gives off (1) two inner or adjacent carotid arteries, which, after running forwards and outwards, pass through their corresponding pituitary spaces ; and (2) two arteries for the supply of the inner or adjacent pseudobranchs. These arteries run at first forwards and dorsal-wards behind the adjacent glossohyals; then curving forwards they unite in front of the adjacent ceratohyals ; then, separating again, they pass between the adjacent palato-quadrates and the hyomandibulars, and are distributed to their corresponding {i. e. inner) pseudobranchs. On either side the first aortic root gives |
