38 HUXLEY, ON THE SKELETON OF FISHES. 
tected to a greater or less extent by lateral roof-like plates, | 
as in the Salmonide ; these Heckel calls Steguri. 3. The 
end of the chorda is enclosed within the anterior cavity of the 
body of the terminal vertebra, as in the Percide, &c. 
I shall bring forward grounds for believing that Heckel is 
mistaken as to this third mode of termination, and that in 
these fishes the end of the chorda really extends far beyond 
the anterior cavity of the last vertebra. 
In 1854, Stannius published (as a part of the new edition 
of the ‘ Handbuch’) his ‘Zootomie der Fische,’ beyond all 
comparison the best and most exhaustive work on the sub- 
ject which has yet appeared. The structure of the fish’s 
tail is discussed at p. 29, but very unaccountably all mention 
of Heckel’s researches is omitted. In the Blennide, Ophio- 
dini, Tenioide, Murenoide, Fistularie, the last caudal ver- 
tebra is said to end ina slight point. In Cyclopterus, Calli- 
onymus, the Pleuronectide, and Plectognathi, “the end of the 
last vertebra becomes flattened and slender, and is prolonged 
into a vertical broad plate, consisting of two quite symme- 
trical halves, an upper and an under.” 
A more detailed (but otherwise essentially similar) account 
to that of Heckel, of the tail of the Salmonide, is next given, 
and the like structure is said to obtain throughout life in the 
Ganoidei, in Esox, Hyodon, &ec., while it is transitory in 
Cyprinide, Characine, and others. 
In conclusion, Stannius points out that ‘ many fish which 
pass for homocercal, show unmistakeable traces of original 
heterocercality.” 
Having verified Stannius’s account of the structure of the 
caudal extremity in the salmon, but seeing no reason to 
doubt—what was generally admitted—that other Teleostean 
fish were truly homocercal, I pointed out, in 1855,* that the 
foundation of the doctrine of Vogt and Agassiz was thereby 
destroyed. For Vogt’s observations were made on a sal- 
monoid fish, and a right comprehension of the structure of 
the tail in such fishes showed, that so far from the heterocer- 
eal tail of the embryo becoming homocercal in the adult, the 
tail of the latter was extremely heterocercal, far more so than 
that of many cartilaginous fishes. In fact, all that Vogt had 
really shown was, that the primitively homocercal tail of the 
embryo becomes gradually more and more heterocercal ; and 
he and others had been misled by the apparent homocercality 
of the adult fish imto supposing that the heterocercality be- 
came lost again, whereas, in point of fact, it was only 
disguised. 
* Friday evening meetings of the Royal Institution. 
