GENERAL INTRODUCTION
TO THE VOLUME.
Although some zoologists have recently revived the old belief that the sponges and the cœlenterates are closely allied, no one in recent times has suggested that there is any morphological relationship between either of these groups and the polyzoa. Personally I do not think that any one of the three groups is allied to any other so far as anatomy is concerned; but for biological reasons it is convenient to describe the freshwater representatives of the three groups in one volume of the "Fauna."
Indeed, I originally proposed to the Editor that this volume should include an account not only of the freshwater species, but of all those that have been found in stagnant water of any kind. It is often difficult to draw a line between the fauna of brackish ponds and marshes and that of pure fresh water or that of the sea, and this is particularly the case as regards the estuarine tracts of India and Burma.
Pelseneer[[A]] has expressed the opinion that the Black Sea and the South-east of Asia are the two districts in the world most favourable for the study of the origin of a freshwater fauna from a marine one. The transition in particular from the Bay of Bengal, which is much less salt than most seas, to the lowerreaches of the Ganges or the Brahmaputra is peculiarly easy, and we find many molluscs and other animals of marine origin in the waters of these rivers far above tidal influence. Conditions are unfavourable in the rivers themselves for the development and multiplication of organisms of many groups, chiefly because of the enormous amount of silt held in suspension in the water and constantly being deposited on the bottom, and a much richer fauna exists in ponds and lakes in the neighbourhood of the rivers and estuaries than in running water. I have only found three species of polyzoa and three of sponges in running water in India, and of these six species, five have also been found in ponds or lakes. I have, on the other hand, found three cœlenterates in an estuary, and all three species are essentially marine forms, but two have established themselves in ponds of brackish water, one (the sea-anemone Sagartia schilleriana) undergoing in so doing modifications of a very peculiar and interesting nature. It is not uncommon for animals that have established themselves in pools of brackish water to be found occasionally in ponds of fresh water; but I have not been able to discover a single instance of an estuarine species that is found in the latter and not in the former.
For these reasons I intended, as I have said, to include in this volume descriptions of all the cœlenterates and polyzoa known to occur in pools of brackish water in the estuary of the Ganges and elsewhere in India, but as my manuscript grew I began to realize that this would be impossible without including also an amount of general introductory matter not justified either by the scope of the volume or by special knowledge on the part of its author. I have, however, given in the introduction to each part a list of the species found in stagnant brackish water with a few notes and references to descriptions.
Biological Peculiarities of the Sponges, Cœlenterates, and Polyzoa of Fresh Water.
There is often an external resemblance between the representatives of the sponges, cœlenterates, and polyzoa that causes them to be classed together in popular phraseology as "zoophytes"; and this resemblance is not merely a superficial one, for it is based on a similarity in habits as well as of habitat, and is correlated with biological phenomena that lie deeper than what are ordinarily called habits. These phenomena are of peculiar interest with regard to difficult questions of nutrition and reproduction that perhaps can only be solved by a close study of animals living together in identical conditions and exhibiting, apparently in consequence of so living, similar but by no means identical tendencies, either anatomical or physiological, in certain directions.
One of the most important problems on which the study of the sponges, cœlenterates, and polyzoa of stagnant water throws light is that of the production of resting buds and similar reproductive bodies adapted to withstand unfavourable conditions in a quiescent state and to respond to the renewal of favourable conditions by a renewed growth and activity.
Every autumn, in an English pond or lake, a crisis takes place in the affairs of the less highly organized inhabitants, and preparations are made to withstand the unfavourable conditions due directly or indirectly to the low winter temperature of the water: the individual must perish but the race may be preserved. At this season Hydra, which has been reproducing its kind by means of buds throughout the summer, develops eggs with a hard shell that will lie dormant in the mud until next spring; the phylactolæmatous polyzoa produce statoblasts, the ctenostomatous polyzoa resting-buds ("hibernacula"), and the sponges gemmules. Statoblasts, hibernacula, and gemmules are alike produced asexually, but they resemble the eggs of Hydra in being provided with a hard, resistant shell, and in having the capacity to lie dormant until favourable conditions return.