C Alternation Of Generations. In the case of the Hydra and the sea-mat, which we have considered above, fresh zooids are produced by a primordial organism by gemmation; the beings thus produced (as well as the parent) being capable not only of repeating the gemmiparous process, but also of producing new individuals by a true generative act. We have now to consider a much more complex series of phenomena, in which the organism which is developed from the primitive ovum produces by gemmation two sets of zooids, one of which is destitute, of sexual organs, and is capable of performing no other function than that of nutrition, whilst the other is provided with reproductive organs, and is destined for the perpetuation of the species. In the former case the produced zooids all resembled each other, and the parent organism which gave rise to them; in the latter case, the produced zooids are often utterly unlike each other and unlike the parent, since their functions are entirely different.

The simplest form of the process is seen in certain of the Hydroid Polypes, such as Hydractinia. The embryo of Hydrac-tinia is a free - swimming ciliated body, which, after a short locomotive existence, attaches itself to some submarine object, develops a mouth and tentacles, and commences to produce zooids like itself by a process of continuous gemmation. These remain permanently attached to one another, with the result that a compound organism is produced, consisting of a number of zooids, or "polypites," organically connected together, but enjoying an independent existence. None of the zooids, however, are provided with sexual organs; and though there is theoretically no limit to the size which the colony may reach by gemmation, its buds are not detached, and the species would therefore die out, unless some special provision were made for its preservation. Besides these nutritive zooids, however, other buds are produced which differ considerably in appearance from the former, and which have the power of generating the essential elements of reproduction. These generative zooids derive their nourishment from the materials collected by the nutritive zooids, but only live until the ova are matured in their interior and liberated, when they disappear. The ova thus produced become free-swimming ciliated bodies, such as the one with which the cycle began.

In this case, therefore, the "individual" consists of a series of nutritive zooids, collectively called the "trophosome," and another series of reproductive zooids, collectively called the "gonosome," the entire series remaining in organic connection.

In other Hydroid Zoophytes allied to the preceding (such as Clytia), the process advances a step further. In Clytia, the generative buds or zooids do not produce the reproductive elements as long as they remain attached to the parent colony; but they require a preliminary period of independent existence. For this purpose they are specially organised, and when sufficiently mature they are detached from the stationary colony. The generative zooid now appears as an entirely independent being, described as a species of jelly-fish (or Medusa). It consists of a bell-shaped disc, by means of which it is enabled to swim freely; from the centre of this disc depends a nutritive process, with a mouth and digestive cavity, whereby the organism is able to increase considerably in size. The substance of the disc is penetrated by a complex system of canals, and from its margin hangs a series of tentacular processes. After a period of independent locomotive existence, the Medusa attains its full growth, when it develops ova and spermatozoa. By the contact of these, embryos are produced; but these, instead of resembling the jelly-fish by which they were immediately generated,, proceed to develop themselves into the fixed Hydroid colony by which the Medusa was originally produced.

Still more extraordinary phenomena have been discovered in other Hydrozoa, as in many of the Lucernarida. In these the ovum gives rise to a locomotive ciliated body, which ultimately fixes itself, becomes trumpet-shaped, and develops a mouth and tentacles at its expanded extremity, when it is known as the "hydra-tuba," from its resemblance to the freshwater polype, or Hydra. The hydra-tuba has the power of multiplying itself by gemmation, and it can produce large colonies in this way; but it does not obtain the power of generating the essential elements of reproduction. Under certain circumstances, however, the hydra-tuba enlarges, and, after a series of preliminary changes, divides by tranverse fission into a number of segments, each of which becomes detached and swims away. These liberated segments of the little hydra-tuba (it is about half an inch in height) now live as entirely independent beings, which were described by naturalists as distinct animals, and were called Ephyrae. They are provided with a swiming-bell, or "umbrella," by means of which they propel themselves through the water, and with a mouth and digestive cavity. They now lead an active life, feeding eagerly, and attaining in some instances a perfectly astonishing size (the Medusoids of some species are several feet in circumference). After a while they develop the essential elements of reproduction, and after the fecundation and liberation of their ova they die. The ova, however, are not developed into the free-swimming and comparatively gigantic jelly-fish by which they were immediately produced, but into the minute, fixed, sexless hydra-tuba.

We thus see that a small sexless zooid, which is capable of multiplying itself by gemmation, produces by fission several independent locomotive beings, which are capable of nourishing themselves and of performing all the functions of life. In these are produced generative elements, which give rise by their development to the little fixed creature with which the series began.

To the group of phenomena of which the above are examples, the name "alternation of generations" was applied by Steen-strup; but the name is not an appropriate one, since the process is truly an alternation of generation with gemmation or fission. The only generative act takes place in the reproductive zooid, and the production of this from the nutritive zooid is a process of gemmation or fission, and not a process of generation. The "individual," in fact, in all these cases must be looked upon as a double being composed of two factors, both of which lead more or less completely independent lives, the one being devoted to nutrition, the other to reproduction. The generative being, however, is in many cases not at first able to mature the sexual elements, and is therefore provided with the means necessary for its growth and nourishment as an independent organism. It must also be remembered that the nutritive half of the "individual" is usually, and the generative half sometimes, compound - that is to say, composed of a number of zooids produced by continuous gemmation; so that the zoological individual in these cases becomes an extremely complex being.

These phenomena of so-called "alternation of generations," or "metagenesis," occur in their most striking form amongst the Hydrozoa; but they occur also amongst many of the intestinal worms (Entozoa), and amongst some of the Tunicata (Molluscoida).