104 Introduction. FISHES. Classification. 
who, unable to draw up a recognizable description, have 
whetted the curiosity of naturalists, but have left the 
species which chance had placed in their hands to hold 
an uncertain niche in our systems. 
A general acquaintance with the principal facts of 
ichthyology is of national importance to a maritime 
country like Great Britain, with an increasing population 
and a limited area of arable land. At the present time, 
the cost of fish renders it rather a luxury for the rich than 
food for the masses ; but there is no doubt that, not- 
withstanding the number of people at present employed 
in catching, curing, and carrying fish to the markets, 
this source of food might be incalculably extended. We 
are still behind the Dutch and Biscayans in some of 
the fisheries carried on in the narrow seas ; and Europe 
at large is vastly surpassed by the Chinese in the 
skill with which supplies for a teeming population are 
extracted from the ocean, or in the economical rearing 
of fish in everj" pond ; a sheet of water in China being- 
made as productive of food as the same extent of good 
arable land. Of late years cod-liver oil, having been 
ascertained to be a most valuable remedy against the 
national malady consumption, has already become an 
extensive article of traffic ; and very recently, experi- 
ments made in France have shown that oil obtained 
from the livers of sharks is not less useful in the same 
disease. Now in March, 1858, when an unbroken 
scull of one kind of shark (the Picked Dog-fish) beset 
the northern coasts of Scotland, and extended twenty 
miles out to sea, filling every harbour and bay — had 
proper fisheries been established, not only might an 
incredible quantity of high-priced oil have been pro- 
cured, but vast stores of sharks’ fins might have been 
prepared for export to China, where they are in great 
request for the purpose of making excellent soup. It 
is, perhaps, too much to expect that Englishmen should 
lay aside their prejudices, and learn from the Chinese 
the modes of converting parts of these fish rejected by 
our fishermen as useless, into palatable and nourish- 
ing food. Not from the observations of our own 
seamen, but from researches made by order of the 
Dutch government, have we learnt that the Herring- 
fishery is productive in those parts of the sea only, 
whose temperature ranges between 54° and 58° of 
Fahrenheit’s thermometer. 
Agassiz, a most important authority in classification, 
has carried the subdivision of the vertebrate animals 
further than his predecessors, and distributes the 
Fishes, as defined above, into four classes, viz : — 
1st. Myzontes, composed of two orders, Myxinoids 
and Cyclostomes. 
2nd. Fishes Proper, of two orders, Cterwids and 
Cycloids. 
3rd. Ganoids, of three orders, Coelacanths (fossil), 
Acipenseroids, and Sauroids, with the addition of o-ther 
three orders, whose true position is to be ascertained 
by future research : these are Siluroids, Plectognailis, 
and Lojjhobranclis. 
4th. Selachians, of three orders, Chimceroids, 
Galeodes, and Batides. This arrangement, considered 
to be in part provisional, and depending on the result 
of investigations now in progress, is not adopted in 
the ensuing pages, though occasionally referred to ; 
the fishes being treated in this work as a single class, 
having a common character — the oxygenation of the 
hlood hy air diffused in water. 
Professor Owen has recently taken a very dlffereni 
-view of the arrangement of the inferior vertebrals from 
the above, and proposes to unite the Fishes, Amphi- 
bians, and Keptiles into one class, which he names 
HiEMATOCRYA. 
“ Our little systems have their day ; 
They have their day and cease to he ; 
They are but broken lights of Tltee, 
And thou, 0 Lord ! art more than they.” 
— Tennyson, In Mem. vi. 
That a correct knowledge of the structure and 
development of any group of animals is as necessary 
to the zoologist as an acquaintance with their phy- 
siognomy and habits, is so evident, that it seems 
unnecessary to affirm that no one can hope to become 
an accomplished ichth 3 mlogist unless he submits to the 
labour of patient anatomical investigation. F or reasons 
already assigned, however, the notices of structure will 
not be extended in this work beyond what is requisite 
for explaining the terms used in characterizing the 
orders and family groups. 
Scales. — In some fishes scales are not developed, 
the skin being generally in that case thick, smooth, 
and slippery, owing to the abundance of defensive 
mucus secreted by its numerous gland.s. Comparative 
anatomists distinguish the internal more or less firm 
framework by the name of endo-skeleton, from the 
external or exo-slceleton, which is intimately connected 
with the skin or developed in its textures, and is 
therefore sometimes called the dermal skeleton. The 
Myzontes of Agassiz, or Dermopleres, furnish an instance 
of an entire order (or, as the author just named con- 
siders them to be, a whole class) destitute of scales, and 
with so little approach to the secretion of bony matter 
that systematic authors call the exo-skeleton “ muco- 
dermoid,” or indurated mucus secreted from the skin, 
the internal skeleton at the same time being cartilagi- 
nous or membranous. Almost all the other great gi-oups 
of fishes contain some species without scales, but allied 
by the rest of their structure to the scaly fishes with 
which they are classed. Scales, when present, originate 
in a little pouch of the external skin, technically named 
a follicle, and resembling the fold of the gum from 
which the germ of a tooth is evolved. Agassiz, when 
studying fossil fishes, was led to distinguish four kinds 
of scales, which he termed cycloid, ctenoid, ganoid, and 
placoid. 
Cycloid scales are smooth on their discs and edges, 
but when examined through a lens, exhibit numerous 
concentric lines, variously flexed, yet having always 
a certain relation to the border of the disc. Most 
generally in that part of the scale which remains in 
the follicle, and is covered by the scale immediately 
before it, these lines of structure by dipping and undu- 
lating produce furrows radiating like the sticks of a 
fan. In all the most important groups of those fishes 
which are clothed with cycloid scales, the bones of the 
skeleton contain radiated or fusiform bone corpuscles.* 
* Kblliker, Proceedings of the Royal Society, ix. p. 656. 
