1895.] 



MALFORMATIONS AMONGST FISHES. 



427 



this is because it is more common for the point of union to lie in 

 front of the anterior limit of the yolk-sac than behind that ap- 

 pendage. When the separation between the two fishes extends 

 further back, some form or another of torsion occurs, due to the 

 gradual shrinkage of the sac, by which the relations between the 

 two fishes are altered. Thus, if the separation extends to the 

 posterior border of the sac only, the caudal end of the monster 

 being single, that part will lie \\ith its dorsal and renrral surfaces 

 upwards and downwards, whilst the anterior parts will lie on their 

 sides with their ventral aspects drawn to« ards one another bv the 

 contraction of the yolk-sac (see fig. 4). If, again, the separation 

 is complete between the two fishes, the condition being that of 

 anakatadidymus, both fishes will finally lie venter to venter in 

 their entire length. Finally, where the separation of the caudal 

 ends has been only temporary, as in the forms included in class 8, 

 the cephalic ends, which are separate from one another, will lie with 

 their ventral surfaces opposed, whilst the caudal ends are thrown 

 one over the other and united in that position. From an examin- 

 ation of the numerous cases figured and described at all ages, it 

 would appear that the embryos always at their earliest period of 

 development lie side by side and that the subsequent changes in 

 position of a part or the whole extent of their bodies are due to 

 the influence of the contraction of the yolk-sac as it gradually 

 becomes emptied of its contents. 



2. Imperfections in one or both members of the double mon- 

 strosity. It has been already mentioned that in cases of parasitism 

 the appendage is often very imperfectly developed, especially in 

 connection with its mouth and eyes, and the same is true even in 

 those instances in which there is no very remarkable difference in 

 size between the two members. The following table of instances 

 examined by myself will show this point quite clearly so far as 

 regards the eyes : — 



Larger fish. 



Smaller fish. 



In several instances also there is a considerable degree of mal- 



