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Dr. J. F. Gemmill. The Anatomy of 



bifid anteriorly. The notoehords remain separate for at least thirty 

 somites, and have the same arrangement of neural and costal arch 

 cartilages as was described in connection with Type la. 



Heart, &c. — The heart chambers and the truncus arteriosus are 

 single, and there are the usual number of gills and gill vessels. There 

 are, however, two sets of carotid and hyoid arteries, the inner or 

 adjacent pairs being derived directly from the truncus arteriosus. 

 The truncus arteriosus arches dorsally in the septum between the two 

 mouths to reach the base of the skull, and then divides into two limbs 

 which are continued backwards to join the aortic collecting roots on 

 either side. The dorsal aorta remains double so long as the notochord 

 is double. 



Head Kidney. — There is a large composite glomerulus containing two 

 vascular tufts and divided into three compartments. Normal Wolffian 

 ducts arise from the outer compartments, while the middle one gives 

 origin to a coiled sacculated tubule which ends blindly in the tissue of 

 the head kidney and represents united adjacent Wolffian ducts. 



The alimentary canal has two mouth openings, two buccal cavities, 

 and two air-bladder diverticula. Pharynx, oesophagus, stomach, liver, 

 intestine, and vent are single. 



Muscles. — In both (a) and (5), so long as the notoehords are separate, 

 there exists between and ventral to them a median muscular mass, 

 divided into segments corresponding with the mesoblastic somites, 

 innervated by the small extra ventral spinal roots previously mentioned, 

 and representing united adjacent lateral muscles. 



Type 2. Union in Pectoral Region. 



(a.) Adjacent Pectoral Fins absent. 



(b.) Adjacent Pectoral Fins present, but united. 



In both cases the brains, the cranial and visceral skeletons, the 

 organs of sense, and the upper parts of the spinal cords are completely 

 doubled. There are two hearts and two trunci arteriosi. In (a) the 

 auricles communicate, and the sinus venosus is a large common chamber 

 receiving two sets of jugular veins, but receiving only a single pair of 

 cardinals. In (b) the auricles are separate, the sinus venosi have only 

 a narrow neck of communication, and there are two complete sets of 

 jugular and cardinal veins. The inner or adjacent set of cardinals is, 

 however, much reduced in size. 



Pectoral Fins. — In (a) pectoral fins are entirely absent from the 

 adjacent sides of the twin bodies ; in (b) they are present in a more or 

 less united condition, the union being greatest towards the posterior 

 border. 



The head kidney resembles that described for Type 1 (b) ; the median 

 tubule is, however, larger, and is continued further backwards. 



