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 _Chimæra_. 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." The five classes above-mentioned together constitute the highest of those sub-kingdoms into which the whole animal kingdom itself is divided. This highest sub-kingdom is named VERTEBRATA, and is called the vertebrate sub-kingdom, because every creature which belongs to it possesses a "spinal column," which is generally built up of bones, each of which is called a "_Vertebra_." We ourselves are members of the genus _Homo_, of the family _Hominidæ_, of the order _Primates_, of the class _Mammalia_, of the sub-kingdom _Vertebrata_, and it is desirable to treat this sub-kingdom at considerable length, both because it is, to us who are members of it, the most interesting and important, and because, by treating it somewhat fully, a good example can be once for all given of biological classification.