Alternations of generations.
Alternation of generations is of common occurrence amongst the Hydrozoa, and something analogous to it has been found to take place in Fungia amongst the Actinozoa. It is not known to occur in the Ctenophora.
The chief interest of its occurrence amongst the Hydromedusæ and Siphonophora is the fact that its origin can be traced to a division of labour in the colonial systems of zooids so characteristic of these types.
In the Hydromedusæ an interesting series of relations between alternation of generations and the division of the zooids into gonophores and trophosomes can be made out. In Hydra the generative and nutritive functions are united in the same individual. The generative swellings in these forms cannot, as has been ably argued by Kleinenberg, be regarded as rudimentary gonophores, but are to be compared to the generative bands developed in the Medusæ around parts of the gastro-vascular system. A condition like that of Hydra, in which the ovum directly gives rise to a form like its parent, is no doubt the primitive one, though it is not so certain that Hydra itself is a primitive form. The relation of Hydra to the Tubularidæ and Campanularidæ may best be conceived by supposing that in Hydra most ordinary buds did not become detached, so that a compound Hydra became formed; but that at certain periods particular buds retained their primitive capacity of becoming detached and subsequently developed generative organs, while the ordinary buds lost their generative function.
It would obviously be advantageous for the species that the detached buds with generative organs should be locomotive, so as to distribute the species as widely as possible, and such buds in connection with their free existence would naturally acquire a higher organization than the attached trophosomes. It is easy to see how, by a series of steps such as I have sketched out, a division of labour might take place, and it is obvious that the embryos produced by the highly organized gonophores would give rise to a fixed form from which the fixed colony would be budded. Thus an alternation of generations would be established as a necessary sequel to such a division of labour. To test the above explanation it is necessary to review the main facts with reference to alternations of generations amongst the Hydromedusæ.
Hydromedusæ[87]. In many instances amongst the Tubularidæ, Sertularidæ and Campanularidæ medusiform buds are produced which become detached and develop sexual organs.
Such Medusæ are divided into two great groups, the Ocellata and Vesiculata, according to the characters of the marginal sense organs. In the Ocellata the sense organs have the form of eyes, and in the Vesiculata of auditory vesicles. The latter seem to be usually budded off from the Campanularia stocks, and the generative organs extend in folded bands over the radial canals. These bands have been regarded by Allman as composed of rudimentary gonophores, and he called the Medusæ which give rise to them blastochemes. He regards them as representing a more complicated type of alternation of generations with three instead of two generations in the series. The Hertwigs have brought what appear to me conclusive grounds for rejecting this view, and have demonstrated that the generative organs of these types resemble those of ordinary Medusæ.
In many forms the medusiform buds though fully developed do not become detached; whether detached or not they are known as phanerocodonic gonophores. In other forms again buds which begin as if they were going to form Medusæ never reach that condition but remain permanently in an undeveloped state. They have been called by Allman adelocodonic gonophores.
In all the above cases two generations at the least interpose between the successive sexual periods, viz.:—
(1) A trophosome produced directly from the ovum.
(2) A gonophore budded from this.