All the insects above mentioned, save the House-fly, have four wings, or else none; but that familiar form may serve as the type of the two-winged order (_Diptera_) to which belong all flies and gnats--including, of course, the Mosquito--and the numerous "Bots," one of which (the Tsee-Tsee fly) is so fatal to cattle in Africa. Finally, amongst insects may be mentioned the wingless, but active order of fleas (_Aphaniptera_), the wingless but sluggish lice (_Aptera_), and the jumping and wingless springtails (_Thysanura_). In leaving the class of insects, we leave 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.