Symmetrical Double Monstrosities in the Trout. 133 



Alimentary Canal. — Mouth, pharynx, air bladder and stomach are 

 double. Union takes place in the pyloric region. Liver, intestine and 

 vent are single. 



Type 3. Union by Posterior Part of Body. 



The intestines are united for a greater or less distance forwards from 

 the vent, which is almost always single. The sagittal planes of the 

 twin bodies converge ventrally in a degree which, roughly speaking, 

 varies directly as the degree of duplicity. The spinal cords may or 

 may not unite anterior to the place of union of the notochords. In 

 some cases the spinal cords remain separate along their whole length. 

 As a rule, in cases where ventral convergence of the sagittal planes is 

 well marked, dorsal structures, such as the spinal cords, dorsal fins, and 

 dorsal edge membranes, remain double longer than structures which are 

 more ventrally placed. 



The twin head kidneys are quite separate, and each gives origin to 

 two Wolffian ducts. The relations of the posterior parts of these ducts 

 and of the bladders show remarkable variety. In rare cases the two 

 adjacent Wolffian ducts (i.e., left duct of right twin and right duct of 

 left twin) end blindly and separately, while the two outer ducts open 

 into a single normal bladder. In all other cases there are two bladders, 

 each of which receives a right and a left Wolffian duct belonging to 

 different twins. The two bladders may be quite separate, or they may 

 communicate with one another. When they are separate each of them 

 may open by a urinary pore, or one of them may have no outlet, and 

 may be greatly enlarged through retention. When the bladders 

 communicate with each other, only one of them possesses a urinary 

 pore. 



The intestines are separate in front, but in all my specimens they 

 unite posteriorly. The united part usually ends by a single vent, but 

 in one remarkable instance two vents were present which terminated 

 by anal orifices situated on opposite sides of the composite body of the 

 monstrosity. 



Type 4. Union by Yolk-sac only. 



Each embryo has a complete and separate complement of organs. 

 The alimentary canals are shut off altogether from one another and 

 from the yolk. The vitelline circulations are crossed. 



General. 



The general part of the paper discusses briefly — 



(1.) The idiosyncrasies and general arrangement of mesial and 

 paired organs at the transitional region in symmetrical double 

 monstrosities. 



