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The Contemporary Review, Volume 36, September 1879

The animal referred to is the lancelet Amphioxus


Every

one knows that frogs begin their existence in the water as tadpoles, which have the habits and mode of life of fishes. Thus, the class _Batrachia_ naturally conducts us to the class _Pisces_, the class of true fishes. This class contains a prodigious variety of forms, and is far more rich in species than any other of the classes before enumerated--even that of birds.

The fishes most familiar to us--such as the perch, carp, mackerel, cod, herring, sole, turbot, salmon, pike, dory, and eel--all belong to one great order called _Teleostei_, and which is made up of what are called "bony" fishes, though there are some bony fishes which do not belong to it. To the same order also belong the Muroena, the electric eel (_Gymnotus_), the flying fishes (_Exocetus_ and _Dactyloptera_), the sucking fish (_Remora_), the pipe-fish and sea-horse (_Hippocampus_), the diodon, the ostracion, the file-fish (_Balistes_), the largest of all fresh-water fishes (_Sudis gigas_ of South America), with a multitude of other forms.

Certain more or less singular Teleosteans are classed together in a subordinate group of "Siluroids" (of which fish the _Silurus_ is a type), and which group includes, amongst others, the singular, cuirassed fish Callichthys.

A group of fishes, which is now very small, but which at an earlier period of the world's history was very large, includes within it all those fishes which

will be hereinafter occasionally spoken of as "Ganoids," as they compose the order _Ganoidei_. Of all the forms of this order, the sturgeon is that which is least unfamiliar to us. The Ganoids are mostly fresh-water fishes and consist of the spoonbill-fish (_Polyodon_), the bony-pike (_Lepidosteus_), the African _Polypterus_, the mud fish (_Lepidosiren_), and the curious Australian fish _Ceratodus_, which last is a singular instance of piscine survival.

Another order, _Elosmobranchii_, is made up of the sharks, together with the skates (or rays) and the curious _Chimaera_. Amongst the skates may be mentioned the celebrated torpedo or electric ray.

The three groups above enumerated contain almost all known fishes, but a few other kinds, all of lowly organization, constitute two other groups of very different structure.

One of these groups is called _Marsipo-branchii_, and contains the lamprey, the _Myxine_ (or Glutinous Hag), and the _Bdellestoma_. They are fishes of parasitic habits and of relatively inferior structure.

Last of all comes a creature of such exceptional build, so widely different from, and so greatly inferior to, any kind of animal yet noticed, that it may but doubtfully be reckoned as a fish at all. The animal referred to is the lancelet (_Amphioxus_), which is a small, almost worm-like animal, living in the sand on our own coasts, and also widely distributed over other parts of the world. The _Amphioxus_ has no distinct head or heart, and its breathing apparatus--its gill structure--differs so much from that of all other fishes as to give a name to its "order" (which contains it alone)--the order _Pharyngobranchii_.

We have now, then, hastily surveyed no less than five "classes" of animals--(1) Mammalia, (2) Aves, (3) Reptilia, (4) Batrachia, and (5) Pisces.

But, as was said in the first beginning of this Essay,[15] "classes" are the groups into which "sub-kingdoms" are divided, and which, by their union, make up such "sub-kingdoms."


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