12 DOUBLE MONSTROSITY— STRUCTURE, CLASS I. 



the fused parachordal cartilages. Its posterior end still shows a rapidly narrowing upturned 

 " heterocercal " portion. In the position of each future vertebra there are four cartilaginous nodules, 

 placed respectively at the dorso-lateral and ventro-lateral corners of the notochord, and prolonged 

 into processes for the neural and haemal arches. 



Nervous System, Heart, etc. The anatomy of the central nervous system and of the organs of 

 special sense, and of the heart and blood-vessels, is, with certain differences in the relative size of 

 parts, practically the same as in the adult condition. With regard to the aortic roots, it may be stated 

 that the first root, i.e. the first branchial vein, gives off the hyoid and carotid arteries and then 

 passes backwards to join the second root. The resulting trunk bends inwards to the middle line, 

 and, meeting with its fellow from the opposite side, forms the upper part of the aorta. This part is 

 next joined on either side by a trunk formed by union of the third and fourth roots. The carotids 

 pass forward beneath the parachordals and, traversing the pituitary space from below, reach the base 

 of the brain. The hyoid artery arises from the first aortic root at its ventral end, perforates the 

 hypohyal, runs up along the hyoid bar, and, after passing through a foramen in the hyomandibular, 

 is continued mainly into the pseudobranch. The efferent vessel of the pseudobranch passes forwards 

 and inwards, traverses the pituitary space from below, and, after running alongside the optic nerve, ends 

 in the choroidal gland of the eye. Of the two posterior cardinal veins, the left is usually the larger. 



Kidneys. The head-kidney, or pronephros, contains a single median glomerular tuft of con- 

 siderable size, supplied by a branch directly from the aorta. The Wolffian ducts begin by a 

 funnel-shaped opening from the glomerular cavity on either side. Then, bending forwards, they 

 become convoluted, and are embedded in highly vascular lymphoid tissue. They next arch back- 

 wards, remaining convoluted for a short distance, and end in a small urinary bladder. The 

 mesonephros is just beginning to develop in connection with their middle and posterior parts. The 

 urinary bladder opens by a mesial pore situated just behind the vent. 



The intestinal canal is completely shut off from the yolk-mass, and there is an open 

 diverticulum for the air-bladder. 



Body Segments. The total number of body segments is approximately sixty, the vent being at, 

 or near, the 36th segment. The posterior edge of the dorsal fin reaches to the 9th or 10th 

 segment in front of the vent, and the anterior border of the adipose fin to the 7th segment behind 

 that opening. 



Median Fins. The median fins characteristic of the adult, viz. dorsal, adipose, caudal, 

 and anal, are already well defined. These are still connected, by a low membranous ridge, 

 which is best marked between the caudal fin on the one hand, and the adipose and anal fins on the 

 other. A similar ridge is found between the anal fin and the vent, and also in front of the vent to 

 near the level of the pelvic fins. In tracing the continuity of the mid-dorsal and mid-ventral lines 

 along the transitional region in double monstrosities, it will occasionally be preferable to speak 

 simply of dorsal and ventral edge membranes rather than to specify the particular fins. 



ANATOMY OF DOUBLE MONSTROSITIES. 1 



CLASS I. 



Union in Head Region, the Ttvin Brains uniting at the Optic Lobes. 



PL I. figs. 4, 5 (external appearance); Pis. VIII.-IX. figs. 35-38 (transverse sections); Pis. 

 X.-XII. figs. 39-46 (horizontal sections); diagrams of skeleton, vessels, etc., in PI. XVII. figs. 58, 59, 

 XVIII. figs. 64, 67, 70, 72, XX. figs. 84, 85. 



In this Class, the region of transition from the double to the single condition involves the 

 brain, the cranial nerves, and the organs of special sense, as well as the cranial, mandibular, and 

 branchial cartilages. The twin heads are placed symmetrically, side by side, and lie in the same 

 horizontal plane. Union is thus of the simple lateral type. 



1 See note at foot of p. 1] . 



