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 highlyorganized 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 guineaworm (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 Planaria (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."

Certain minute organisms, familiarly known as "Wheel-Animalcules," or Rotifers, form the "class" Rotifera. They have gained their name through an apparently (though, of course, not really) rotary motion, of that end of their bodies at which the mouth is situated. Here also may be mentioned certain curious aquatic worms called Gasterotricha, which are closely allied to the wheel animalcules.

Finally may be mentioned the class Gephyrea, containing animals, worm-like indeed in form, but which have much apparent affinity to the group next to be spoken of-the group of star-fishes and their allies. Amongst the Gephyrea may be mentioned the worms called Sipunculus and Priapulus.

This leads us to the sub-kingdom containing the star-fishes-the sub-kingdom ECHINODERMA, which includes, besides the star-fishes (or Asteridea), all sea-eggs or sea-urchins (Echinidea), the brittle-stars Ophiuridea, as well as the elongated soft animals called sea-cucumbers, or Holothuridea, some of which latter are known as the Japanese edible, "Trepang."

Besides these groups there are still surviving a few creatures (Comatula and Pentacrinus) belonging to the class of "sea-lilies," or Crinoidea, reatures which once lived in countless multitudes, but have now almost entirely passed away. All these crinoids were like star-fishes on stalks, and of the existing forms, Pentacrinus still passes the whole of its life, and Comatula its youth, in a stalked condition.

The next great primary division, or sub-kingdom of animals, is CELENTERA, and a good type of the cœlenterates, the sea anemone (Actinia), has now become a familiar object to us in our aquaria. These animals are plant-animals, or zoophytes, and some of them build up coral-reefs, or islands, and it is one kind which produces the red coral of commerce. Forms essentially similar, but the solid supporting framework of which is of a softer nature, are such as Alcyonium and Pennatula. All these belong to the "class" Actinozoa. There are other cœlenterates of an active free-swimming habit, such as Beröe and Cydippe, which are balls of glassy transparency displaying iridescent hues as they move rapidly through the water by means of their peculiar locomotive organs.

Other cœlenterates, of the same essential type but of simpler structure, form the class Hydrozoa. Amongst these may be mentioned the little Hydra of our ponds, which will often come before us in our survey of animal life. Some compound forms of Hydrozoa simulate the compound Actinozoa; such are the calcareous millipores, and those with a softer

structure, called "corallines," such as Eudendrium and many others. The Portuguese man-of-war (Physalia) and the various forms of jelly-fish (Meduse) all belong to the Hydrozoa, as also does a very curious and very elementary form, to which the name Tetraplatia has been given.

Next we come to the group of sponges, SPONGIDA, some of which— as the now well-known Euplectella-are of marvellous beauty and delicacy of structure; while others, as the sponge of commerce, are of much greater simplicity of form. Simplest of all the sponges is the sponge called Ascetta Primordialis. Some sponges have a horny, some a calcareous, and some a siliceous skeleton, and (strange as it may appear) some have a habit of boring into shells, and living in the excavations they make.

An animal recently discovered, Dicyema, may at this initial stage of our inquiry be left with its place and affinities undetermined. It is a minute worm-like creature of most exceptionally simple structure, which lives parasitically within cuttle-fishes.

We now pass to animals (if so they are really to be considered) which are the lowest and simplest of all, and which are mostly microscopic in size, and may be grouped together under the term HYPOZOA, or under the generally employed name Protozoa. With very few exceptions these animals are aquatic, and if terrestrial they are found in damp localities. Some are marine, others are fresh-water organisms.

The highest of the group are the animalcules, which are named Infusoria, most of which are freely swimming organisms, though a certain number of them live fixed to some supporting body.

Another group of Hypozoa is that termed Gregarinida, a group made up of very lowly parasites, such as are often found tenanting the intestines of insects as well as those of higher animals. Finally, we have the group of Rhizopoda, animals which have the faculty of projecting and retracting (so to say, at will) filamentary or conical processes of their semi-fluid substance, such processes being the Pseudopodia, which were referred to earlier.*

Amongst the Rhizopoda, the most complex and beautiful are the delicate and symmetrical creatures known as Radiolaria,† the siliceous skeletons of which are amongst the most remarkable of microscopic objects.

Allied to them are the simpler Heliozoa, of which the after-mentioned Actinophrys may be taken as a type.

Next come the Flagellata, or minute creatures which swim about by means of one or two whip-like processes, whence the name of the group. Last of all is the group of Foraminifera, animals which are well worthy of note, seeing that, though they are each but as it were a minute particle of structureless jelly, they manage to build most complexlyformed, generally calcareous, shells, or to pick up from the sand of the

See CONTEMPORARY REVIEW, July, p. 710.

For a summary of our knowledge respecting this group, see the "Linnean Society's Journal," Vol. xiv. (Zoology), p. 136.

sea minute particles, which they agglutinate around them with marvellous neatness and precision. Their calcareous shells are generally pierced by a multitude of minute pores, through which the little creatures protrude their pseudopodia. It is from these pores (or foramina) that the group receives its name. All Foraminifera, however, are not provided with shells. Some, as the Amæba, are naked, and the simplest of all animals, Protogenes and Protamoba, consist of but a minute particle of semi-fluid jelly, or protoplasm, naked and as devoid of every external protection as it is of internal organization.

We have thus descended to the bottom of the animal kingdom, and passing from these rudimentary forms, which are generally reckoned as animals, we may next survey in ascending order the different organisms which together compose the kingdom of Plants, a group much less rich in species than is the animal kingdom.

At the bottom of that kingdom are very simple creatures, but little different, to all appearance, from the lowest animals. As an example of such we may take the minute plant Protococcus, which is an humble member of the great group of Alge, to which all sea-weeds belong. Not all of this important tribe, however, are marine. Many are found in fresh water-such as the protococcus itself, and many of the green vegetable threads known as Conferræ. Some even live on land, and draw their moisture from the atmosphere. The Algae are exceedingly varied in their structure; some, like the protococcus, being of extreme simplicity; others attaining a large size, and presenting the appearance of a stout stem with branches and leaves.

The Algae are divisible into the green-spored* (Chlorospermea), the rose-spored (Floride), and the olive-spored (Melanospermeæ).

It is in the first division that the Protococcus may be placed, as also those microscopic plants called Diatoms and Desmids. The former, the Diatomaceæ, are a very numerous group of minute organisms, some of which are used as test objects for microscopes. They contain in their outer coat or case a relatively large portion of silex, and their remains here and there form deposits-vast beds many feet in thickness -known as "tripoli," and used for polishing. The minute particle of their protoplasm is contained within the siliceous case. They may be entirely free, or cohere in aggregations, or be attached to a supporting surface by a slender stalk, which may ramify and bear a little siliceous case or "frustule" at the end of each branch.

The desmids (or Desmidiaceae) are green and devoid of silex, though their protoplasm is enclosed in hard or flexible cases, often marked with beautiful and characteristic patterns.

Both diatoms and desmids may cohere together, forming more complex masses; but another creature allied to Protococcus is noted for its mode of cohesion. This is the microscopic plant Volvox, the individuals of which cohere so as to form spheroidal aggregations, which swim about

* A "spore" is a minute reproductive particle.

by the action of filamentary prolongations of their protoplasm, such prolongations reminding us of the pseudopodia of radiolarians and other rhizopods.

Amongst these simplest plants may be also mentioned the curious thread-like organisms, which, on account of their remarkable and as yet unexplained movements, are called Oscillatoriæ.

Another curious vegetable organism which may here be mentioned is Vaucheria. It is a green, thread-like plant, which may be several inches long, and which at one stage of its existence (when it is what is called a "spore") swims about by pseudopodial prolongations of its protoplasm.

Some few of the Chlorosperme are large and conspicuous organisms. Such, e.g., is Caulerpa, which abounds on warm, sandy coasts, and on which turtles browse. Though, as we shall hereafter see, it is really as simple in structure as a particle of yeast, it yet presents a very complicated external figure.

Some of the great group of Alge attain enormous dimensions. Thus, Macrocystis (one of the Melanosperma), of the Southern Ocean, may be even 700 feet in length. Another kind, Lessonia, forms submarine forests, with stems like the trunks of trees.

The group of Floridia includes the delicate and elegant sea-weeds, which are amongst the most admired vegetable productions of our coasts. They are of interest, on account of various peculiarities in their reproductive processes.

Other lowly plants may, at least provisionally, be placed in the great group to which mushrooms and truffles belong-the group of Fungi—a group the members of which agree in certain exceptional phenomena of function,* as well as of structure and composition-as they are exceptionally nitrogenous.

Amongst the lowest which we may for convenience provisionally include in this group may be mentioned minute Vibrios, such as the Bacteria so much talked of in connexion with spontaneous generation, and the small plant which by its growth produces fermentation-the yeast-plant (Saccharomyces).† Closely allied to the yeast-plant are the "moulds" which grow on organic matters such as Penicillium, Mucor, Saprolegna, Phytophthora, the last of which is the potato disease.

A singular group of organisms goes by the name of Myxomycetes. These enigmatical creatures have been classed in turn as animals and as plants, and, indeed, at one period of their existence they seem to have more resemblance to the former, while at another stage of their life history they must unquestionably be ranked as plants. When young, they are in a semi-fluid condition, and so move that they seem, as it were, to flow over the body on which they rest. They grow

* See CONTEMPORARY REVIEW for July, 1879, p. 714.

+ Some botanists think that yeast is no true and definite kind of plant, but that it is only a conglomeration of fungoid spores of divers sorts.