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 _Planariæ_ (which are mostly fresh-water, though some live on land), allied to the flukes.
The class of tape-worms (_Cestoidea_) is one most numerous in its kinds, which are all completely parasitic in habit. Some of them are so fatal in their effects that they are estimated to occasion every seventh death which occurs in Iceland, and they cause mortality amidst our own flocks, producing in sheep the disease known as the "staggers."