Hydrozoa

“ To these three characters the Hydrozoa add a fourth which is distinctive of the subdivision of the Coelenterata termed the Cnidaria; that is to say, they always possess peculiar stinging organs known as nettle-cells, or nematocysts (Cnidae) , each produced in a cell forming an integral part of the animal’s tissues. ”

“ The marine forms are vastly more numerous, and more easily found, since not only pieces of weed upon which they grow are to be found upon every beach, but whole masses of leaf-like colonies, forming what is known as horn-wrack, may be plentifully found. Instead of the tentacles armed with sting-cells, like the anemone’s, possessed by the Hydrozoa, the Polyzoa have arms clothed with active cilia, by which the food is swept into the mouth, passing on into the stomach, and then through the intestine to another opening. ”

“ Nevertheless there is no other class in the animal kingdom with better marked characteristics, or with more uniform 172 morphological peculiarities underlying the utmost diversity of superficial characters.
All Hydrozoa, in the first place, exhibit the three structural features distinctive of the Coelentera (q.v.) . (1) The body is built up of two layers only, an external protective and sensory layer, the ectoderm, and an internal digestive layer, the endoderm. ”

“ In zoology a hollow process occupying the axis of the generative buds of hydrozoa. Sporosac. Gr. spora, a seed, and sac. The body containing the ova of hydrozoa. Somatic Fluid. Gr. soma, the body. The fluid which contains digested food, and taking the place of blood, circulates through the body of hydrozoa. Tentacles. Lat. tentaculus, a little arm. The arms or prehensile organs of Sea-anemones, &c. Thread Cells. Cells containing an extensible microscopic thread, possessing stinging properties, common among the Cœlenterata. Thorax. Gr. a breastplate. The chest.
[101]
INDEX.
Fig. ”

“ The first specimen I ever possessed of the beautiful confervoid Hydrodictyon utriculatum was obtained in this way—the water, after depositing a green sediment, gave birth to a rich net-work of vegetation, in which this conferva was conspicuous. Several of the jars which I prize the most are those which have been simply filled at the brook side, and left for the minute germs of animal and vegetable life to develope in their own way, and it is not long before a rich growth takes place, that affords abundant material for observation with the microscope. ”

“ The plants have collected stores of organic matter, and their minute germs, along with microscopic species, are floating everywhere in the sea. Nay, there may be abundant examples of those Amœba-like germs of aquatic plants, simulating for a time the life of the animal, and then returning into the circle of vegetable life. In these some might see precursors of the Protozoa, though they are probably rather prophetic analogues than blood relations. The plant has fulfilled its function as far as the waters are concerned, and now arises the opportunity for the animal. ”

“ Further, their mode of feeding may be saprophytic in which nourishment is absorbed by diffusion through the body surface in the form of soluble organic substances, holozoic where solid food particles are taken in, or holophytic in which food is synthesised by the energy of sunlight. This last group is commonly spoken of as the Phyto flagellates, which are to all intents and purposes unicellular algæ, and as such will be dealt with in Chapter VI.
The class CILIOPHORA consists of those protozoa whose typical organs of locomotion are threads or cilia. ”

“ But in the Metazoa and Metaphyta at large, the associated units are, with the exceptions just indicated, completely subordinated. The unit-life is so far lost in the aggregate life that neither locomotion nor the relative motion of parts remains; and neither in shape nor composition is there resemblance to protozoa. Though in many cases the internal protoplasm continues to carry on vital processes subserving the needs of the aggregate, in others vital processes of an independent kind appear to cease. ”

“ In the Metazoa the sexual process is always related to the production of a new individual, that is to say, of a multicellular organism for which there is no analogy amongst the Protozoa, although an approach to the Metazoan condition is seen in colony-forming Flagellata, such as Volvox and its allies. The reproduction of Protozoa is analogous to the ordinary process of cell-division and multiplication which is going on at all times in the bodies of the Metazoa, and which can be observed in the production of the gametes ”

“ This is effected by the osmotic pressure of the internal cell-fluid and the elasticity of the cellulose wall, which is thus expanded. Nevertheless, in both cases—and in all "vital" phenomena—the real cause of the process is, in the ultimate analysis, the chemical play of energy in the active plasm.
The metaphyta, with few exceptions, are fixed in one spot for life, or only mobile for a short time when they are young. In this they resemble the lower metazoa, the sponges, polyps, corals, bryozoa, etc. They have not free locomotion. ”

“ If, as seems to be indubitable, the micro-organisms of the soil are of importance to soil fertility, it is necessary for us to know in what way this population is affected by the application of fertilisers, and a start has been made by investigating the effects of hydrogen ion concentration on soil protozoa. Much has already been written concerning this question, but almost entirely on results obtained in artificial cultures. It is always dangerous to argue from the artificial to the natural environment of organisms and [94] particularly so in respect to the soil. ”

“ The asexual swarm-spores have the peculiarity of never leaving the mother cell. They swim around in the cavity bounded by the cell wall and shortly come to rest, arranging themselves to form a new net entirely within the parent cell. Thus by their habits one may readily distinguish the asexual zoospores and gametes of Hydrodictyon.
Now let us summarize the factors that will make Hydrodictyon form asexual zoospores at one time and gametes at another. The waternet grows luxuriantly in a culture solution containing a number of inorganic salts. ”

“ Finally, as Nasir has shown, the protozoa play a part in the complicated nitrogen cycle, and work of this type needs extending.
Such, then, are a few of the outstanding problems that confront the soil protozoologist; but he must always remember that the organisms he studies are but a small fraction of the total, and that any influence affecting one part of the complex will be reflected in another. As Prof. Arthur Thomson said in his Gifford Lectures, “No creature lives or dies to itself, there is no insulation. ”

“ Dessicated Protophyta and Protozoa shrink into mere dust; and among the acalephes we find but a few grains of solid matter to a pound of water. The higher aquatic plants, in common with the higher aquatic animals, possessing as they do much greater tenacity of substance, also contain a greater proportion of the organic elements; and so are chemically more unlike their medium. And when we pass to the superior classes of organisms—land plants and land animals—we find that, chemically considered, they have little in common either with the earth on which they stand or the air which surrounds them. ”

“ The protozoa, fungi, algæ and bacteria are all microscopic organisms, and are the agents responsible for the decomposition of the organic residues in the soil; they do not act as independent units, the processes of one group being dependent upon and intimately related with those of the others. During the activities of these organisms various organic and mineral substances are decomposed or transformed into materials, such as humus and the inorganic compounds of nitrogen, phosphorus, potassium, etc., necessary or helpful for the growth of plants. ”

“ Converse facts are presented after development has been completed; for with the commencement of reproduction we everywhere see more or less resumption of individual life among the units, or some of them. It is a trait of transitional types between Protozoa and Metazoa to lead an aggregate life as a plasmodium, and then for this to break up into its members, which for a time lead individual lives as generative agents; and sundry low kinds of plants possessing small amounts of structure, have generative elements—zoospores and spermatozoids—which show us a return to unit life. ”

“ These beads and bubbles are pure oxygen, which the plants distil from the water itself, in order to obtain its hydrogen, and from carbonic acid, in order to obtain its carbon.
There is one more feature, which no writer on the aquarium has yet noticed, namely—when a tank is properly stocked, the water soon gets crowded with infusorial animalculæ, which swarm among the plants, and on the sides of the glass in countless thousands, visible only by the aid of the microscope. These are in accordance with a natural law; the presence of vegetable matter in water always induces them. ”

“ The polyps also include the salt water animals called hydroids, jellyfishes, and coral polyps.
Hydroids.—Figure 40 shows a hydroid, or group of hydra-like growths, one of which 29eats and digests for the group, another defends by nettling cells, another produces eggs. Each hydra-like part of a hydroid is called a hydranth. Sometimes the buds on the hydra remain attached so long that a bud forms upon the first bud. Thus three generations are represented in one organism. Such growths show us that it is not always easy to tell what constitutes an individual animal. ”

“ Infusorial animal life, as vorticellæ, stentors, rotifers, and various polyzoa, cling, as also do hydra, in colonies to vallisneria, duck-weed, frogbit, or small branches dipping down under water; and if some of the water-weed is brought home the little creatures will live and thrive for several weeks. No waters, however, are so full of minute animal life as the sphagnum bog. A number of species of diatoms, as well as protozoids and the smaller molluscs, will be found in all peat bogs. It is remarkable, too, that the same species, everywhere, are associated with this kind of moss. ”

“ We might almost claim that if the evolution of man from the single cell be granted, no other order of their occurrence is possible.
The protozoa are mostly, though not purely, nutritive and reproductive. These functions are essential to the existence of the species. Naturally in the early protozoan colonies, and in forms like hydra, these functions predominated. But mere digestive tissue is not enough for digestion. Muscles are needed to draw the [110] food to the mouth, to keep the digestive sack in contact with it, and for other purposes. ”

“ Advance from the homogeneous to the heterogeneous, itself involves an increasing distinction from the inorganic world. Passing over the Protozoa, of which the simplest probably disappeared during the earliest stages of organic evolution, and limiting our comparison to the Metazoa, we see that low types of these, as the Cœlenterata, are relatively simple in their organization; and the ascent to organisms of greater and greater complexity of structure, is an ascent to organisms which are in that respect more strongly contrasted with the structureless environment. ”

“ We may then consider the Metazoa, or many-celled animals, as a polyphyletic, or at any rate diphyletic group, including two perfectly distinct lines of descent from the ancestral Protozoa, the Sponge-line on the one hand, which leads to nothing higher than Sponges, which retain in many respects the characters of Protozoa, and the Enterozoon line on the other, which leads through the Coelenterata to the Coelomata and so on to the highest divisions of the animal kingdom. ”

“ There is the fundamental fact that the plants and animals with which we are familiar—Metaphyta and Metazoa—are formed by the aggregation of units homologous with Protozoa. These units, often conspicuously showing their homology in early embryonic stages, continue some of them to show it throughout the lives of the highest type of Metazoa, which contain billions of units carrying on a protozoon life. Of the protoplasts not thus active the great mass, comparatively little transformed in low organisms, become more and more transformed as the ascent to high organisms goes on ”

“ Hydroids grow in all parts of the ocean, in deep water as well as near the coast. Some of them are three feet high. One branch may contain a hundred thousand distinct animals, the only communication between them being a circulation of fluid through the hollow stems. In this way each branch constitutes a family which has sprung originally from the same little egg. Some varieties never grow tall, and as they occur in patches over rocks and shells, they resemble thick beds of moss. ”

“ The life history of each species has its own characteristic features as regards nuclear division, etc., and in many forms, notably the amœbæ, it is impossible to identify them with certainty unless the chief stages of the life history are known. In general, however, the soil protozoa pass through very similar phases and develop in a perfectly straightforward way. ”

“ Sporozoa in the persistence of the encysted stage are very plant-like, though they are often intracellular and are parasitic in living Animals. On the other hand, the Infusoria for the most part answer to all the physiological characters of the Animal world, but are single cells, and by the very perfection of their structure, all due to plasmic not to cellular differentiation, show that they lie quite off the possible track of the origin of Metazoa from Protozoa. ”