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Show 14 DR. J. F. GEMMILL ON [May 12, 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 resulting vessel is then joined by the third and fourth aortic roots, but it remains separate from its fellow so long as the notochords are separate, i. e. back to the twentieth somite. The arrangement of vessels which has just been described and which is illustrated in PL II. fig. 13 is somewhat remarkable. Mixing of arterial with venous blood must have taken place in no slight degree, and certain parts in each twin head must have been supplied by blood coming directly from the ventral aorta. There is no trace of a pair of adjacent jugular veins, and it is difficult to make out from sections the course of the venous blood coming from the adjacent sides of the twin heads. But as there is a considerable amount of spongy tissue below the base of the skull and in the septum between the mouth-openings, it is probable that the blood in question found its way into the median and the main jugulars. The presence of this spongy tissue no doubt indicates congestion. The pronephric glomerulus (PL IV. fig. 32) is composite. It is remarkably large, and is divided into three compartments by two vascular tufts each of which has an afferent and an efferent vessel. Normal Wolffian ducts arise from the outer compartments, while the middle compartment gives origin to a sacculated tubule which passes backwards a short distance to end blindly, and which represents fused adjacent Wolffian ducts. There are two buccal cavities, but the oesophagus and the rest of the alimentary tract are single except for the presence of two air-bladder diverticula. The composite muscles described on page 9 are well developed in this monstrosity. II. Union in Pectoral Region. Class II. of the monstrosities has next to be described. In it the twin bodies are united in the region of the pectoral fins. All specimens of this Class have separate gullets, air-bladders, and stomachs, but a single liver, intestinal canal, and vent. The hearts are more or less closely united. Two subgroups of this Class are naturally distinguished from one another-(«) a group in which union of the twin bodies takes place so far forward that adjacent pectoral fins are not present; and {b) a group in which adjacent pectoral fins are present, but are united and reduced in size. (a) Adjacent Pectoral Fins not represented.-The brain, the cranial and branchial skeletons are double. So also are the alimentary canals down to the level of the stomach. The chief interest in this type centres round the heart and blood-vessels. A reconstruction drawing of the heart and origins of the vessels of |