eave all the more highly-organized
Invertebrata. But the next group to which we may direct our attention is
one which is exceedingly numerous, and contains a very varied assemblage
of forms. This group is the "sub-kingdom" of Worms, VERMES. First
amongst its contents may be mentioned the higher or true "worms," such
as the earth-worm (_Lumbricus_), the leech (_Hirudo_), the sea-mouse
(_Aphrodite_), and their allies, together with the worms which live in
tubes, which are called _Tubicolous_-"_Annelids_," because the whole
class of these higher worms bears the name _Annelida_.

In this connexion may be mentioned certain exceptional vermiform
creatures, about the affinities of which naturalists dispute.

One of these is a marine creature (called _Sagitta_, from the way in
which it shoots like an arrow through the water), which has many
affinities to Arthropods.

Another is a most remarkable worm, which has been found in the Bay of
Naples, and is called _Balanoglossus_. It is the type of a group called
_Enteropneusta_. To it reference will have again and again to be made on
account of certain singularities in its structure.

A very distinct class of creatures is termed _Bryozoa_ (or _Polyzoa_),
and is composed of very minute animals which live in compound
aggregations, and often grow up in an arborescent manner. The common
sea-mat (_Flustra_) is one example of the class, and another--a good
type--is called _Plumatella_. The _Bryozoa_ have many affinities with
the _Mollusca_, to which some naturalists consider them to belong.

Other worms form the class _Nematoidea_, of which many are parasitic and
many not so. Amongst the better known of the former may be mentioned the
worms which tease children (_Ascarides_), the guinea-worm (_Filaria_),
the scourge of Germans who eat raw meat (_Trichina_), the deadly
blood-parasite of the Nile (_Bilharzia_), and many others.

Another class (_Trematoda_) is made up of parasites called "Flukes," to
some of which (_e.g._, _Monostomum_) reference will have hereafter to be
made with respect to their processes of development.

The class _Turbellaria_ contains a variety of other worms of a lowly
kind, one or two of which (_e.g._, _Borlesia_) live coiled up in complex
tangles which, if unravelled, would attain a length of forty feet.
Amongst the commoner kinds may be mentioned the worm _Nemertes_, and all
worms called _Planariae_ (which are mostly fresh-water, though some live
on land), al