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 _Hominidae_,
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.

But the number of animal kinds which belong to other sub-kingdoms vastly
exceeds the total number of vertebrate animals, and the structural
contrasts found between different non-vertebrate species is very much
greater than any such contrasts as can be found to exist between any two
members of the highest, or vertebrate sub-kingdom. This is only what we
might expect; for non-vertebrate animals--often spoken of collectively
as "_Invertebrata_"--form several distinct sub-kingdoms, each of which
has a rank approximatively co-ordinate with that sub-kingdom to which we