STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
405 
Part IV.— The skeleton of the ventral lobe of the tail fin , and its bearing on the nature 
of the tail fin of the various types of Pisces. 
In the embryos or larvae of all the Elasmobranchii, Ganoidei, and Teleostei which 
have up to this time been studied, the impaired fins arise as median longitudinal folds 
of the integument on the dorsal and ventral sides of the body, which meet at the apex 
of the tail. The tail at first is symmetrical, having a form which has been called 
diphycercal or protocercal. At a later stage, Usually, though not always, parts of these 
fins atrophy, while other parts undergo a special development and constitute the 
permanent unpaired fins. 
Since the majority of existing as well as extinct Fishes are provided with discon¬ 
tinuous fins, those forms, such as the Eel {Anguilla), in which the fins are continuous, 
have probably reverted to an embryonic condition : an evolutional process which is of 
more frequent occurrence than has usually been admitted. 
In the caudal region there is almost always developed in the larvse of the above 
groups a special ventral lobe of the embryonic fin a short distance from the end of the 
tail. In Elasmobranchii and Chondrostean Ganoids the portion of the embryonic tail 
behind this lobe persists through life, and a special type of caudal fin, which is usually 
called heterocercal, is thus produced. This type of caudal fin appears to have been the 
most usual in the earlier geological periods. 
Simultaneously with the formation of the ventral lobe of the heterocercal caudal fin, 
the notochord with the vertebral tissues surrounding it, becomes bent somewhat dorsal- 
wards, and thus the primitive caudal fin forms a dorsally directed lobe of the hetero¬ 
cercal tail. We shall call this part the dorsal lobe of the tail-fin, and the secondarily 
formed lobe the ventral lobe. 
Lepidosteus and Amia (Wilder, No. 15) amongst the bony Ganoids, and, as has 
recently been shown by A. Agassiz,* most Teleostei acquire at an early stage of their 
development heterocercal caudal fins, like those of Elasmobranchii and the Chondro¬ 
stean Ganoids; but in the course of their further growth the dorsal lobe partly 
atrophies, and partly disappears as such, owing to the great prominence acquired by the 
ventral lobe. A portion of the dorsally flexed notochord and of the cartilage or bone 
replacing or investing it remains, however, as an indication of the original dorsal lobe, 
though it does not project backwards beyond the level of the end of the ventral lobe, 
which in these types forms the terminal caudal fin. 
The true significance of the dorsally flexed portion of the vertebral axis was first 
clearly stated by Huxley,! but as A. Agassiz has fairly pointed out in the paper 
already quoted, this fact does not in any way militate against the view put forward by 
* “ On the Young Stages of some Osseous Fishes.—I. The Development of the Tail,” Proc. of the 
American Academy of Arts and Sciences, vol. xiii., 1877. 
t “ Observations on the Development of some Parts of the Skeleton of Fishes,” Quart. Journ. of Micr. 
Science, vol. vii,, 1859. 
