| OCR Text |
Show 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 thau behind that appendage. 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 with its dorsal and ventral surfaces upwards and downwards, whilst the anterior parts will lie on their sides with their ventral aspects drawn towards one another by the contraction of the yolk-sac (see fig. 4). If, again, the separation is complete between tbe 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 examination 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 monstrosity. 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 :- 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Eight. Normal. » • u n » >> ,, Larger fish. Left. Normal. j. >> » a )» n n a »» Smallet Eight. Normal. Choroidal fiss. unclosed. Absent. A pigment spot .°p1y.« ii Absent. Normal. A pigment spot only. Normal. Absent. fish. Left. Absent. Very small. Absent. a Normal. Absent. Lens with slight ring of pigment around it. Absent. A pigment spot only. Absent. In several instances also there is a considerable degree of mal- |
