DOUBLE MONSTROSITY STRUCTURE, CLASS II. 17 



hypohyal pieces of the outer arches. No adjacent elements are interposed in the series of branchial 

 cartilages. Here, 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 

 its articulation with the second branchial cartilages. The succeeding copular pieces are single, but 

 they are a little broader than normal, especially in front. 



Notochords, etc. The notochords remain separate as far back as the twentieth to the twenty- 

 eighth somite. The arrangement of the neural and haemal arch cartilages in the transition region 

 is the same as that described on p. 13. 



Brain and Spinal Cord. 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 inner nerve-roots. In the medulla these 

 roots are extremely rudimentary, and their ultimate distribution could not be traced, but in the 

 spinal cord they 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 muscular mass which lies 

 ventral to the notochords (compare p. 15). 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 the 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, while the 5th are 

 reduced in size. The inner or adjacent 7th and 8th nerves 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 recti muscles makes it probable that 

 these nerves are present. 



Aiiditory Organs. 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 diagram of this labyrinth is given in PI. XX. fig. 82. Utricle, anterior, and posterior 

 semicircular canals are all represented, but there is no trace of a horizontal semicircular canal. 



Heart and Vessels. 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 may be taken to 

 represent a very rudimentary set of inner or 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. 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 dorsalwards behind the adjacent glossohyals. They next curve 

 forwards and unite in front of the adjacent ceratohyals ; then, separating again, they pass between 

 the adjacent palato-quadrates and the hyomandibulars, and are distributed each to its corresponding 

 pseudobranch. On either side the first aortic root gives off (1) a hyoid artery which sends a branch 

 to the corresponding outer pseudobranch, and (2) a carotid artery. It then joins the second aortic 

 root, and shortly afterwards meets the continuation of the ventral aorta previously mentioned. The 

 c 



