The numerous group of bugs is allied to the plant-lice (Aphides), which so often infest our Pelargoniums when kept in dwelling-rooms. Allied to them, again, are the small creatures the nature of which was so long disputed, though familiar to commerce as "Cochineal." Really, they are small, singularly inert, plant-lice, which adhere to the surface of certain "Cacti."
The Dragon-flies, before referred to, are the types of the order Neuroptere.
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.