In Cristatella we have the most highly modified of all the Phylactolaemata. The individuality of the zooecium is here subordinated to that of the colony as a whole. The branched arrangement of the zooecia is greatly obscured. The body-cavities have become completely confluent, although rudiments of the septa still exist. The ectocyst has been lost, with the exception of the basal layer of the colony. The tentacles are more numerous (80-90); and in accordance with the increase in the elaboration of the genus, its statoblasts belong to the most complicated type known.
Fig. 251.—Statoblasts of Phylactolaemata. A, Fredericella sultana Blum., × 38; B, Plumatella repens L., × 38; C, Lophopus crystallinus Pall., × 28; D, Cristatella mucedo Cuv., × 28. (A, from Allman; B-D, from Kraepelin.)
The production of floating statoblasts may seem a strange adaptation to the conditions of fresh-water life, since it might be assumed, a priori, that these structures would be specially liable to be frozen during the winter. The following experiments made by Braem[[568]] show, however, that the germinating power of the statoblasts is improved by a certain amount of frost. A number of statoblasts were taken; half of these were placed in water, which was then frozen; and these were found to germinate readily when afterwards exposed to suitable conditions. The other half were not subjected to the action of frost; and these could not be made to germinate, even although the water had been cooled to a point slightly above the freezing point. It thus appears that the buoyancy, so far from being a risk, is a means of exposing the statoblast to the conditions which are most favourable to its later development.
Braem supposes that the beneficial action of frost is due to a lowering of the vital energy of the statoblast. As in the case of reproductive bodies known in many other fresh-water organisms, the statoblast germinates only after a period of rest. Although this period is often shortened by a lowering of the temperature, it can also be induced by the exclusion of air, as in an experiment during which the statoblasts were enclosed in airtight tubes. The respiratory processes were thereby lessened, and the germinating power was materially improved.
Since the development of the statoblasts depends largely on the temperature, the first warm weather in early spring will probably induce the germination of those which are floating; and the young colony, leaving the protection of the statoblast, will become susceptible to frost. But even if the first-formed colonies are killed off by a subsequent frost, other statoblasts which have remained in the mud during the winter are disentangled from time to time, and germinate on reaching the surface.
Distribution.—The protective value of the shell is also shown by the fact that the statoblast may be kept for some months in a dry condition without losing its power of germination. There can be little doubt that the capability of withstanding desiccation enables the species to enlarge its area of distribution. It is asserted that fresh-water Polyzoa decrease in abundance in proportion to the distance from the mouth of the river in which they are found. The current will naturally tend to bring together the statoblasts from the Polyzoa growing in the upper waters.
Nothing is more surprising than the wide geographical distribution of the Phylactolaemata. The European genera are all recorded from North America. Fredericella, Plumatella, and Lophopus are further recorded from Australia; while Plumatella is known to occur also in Malacca, the Philippine Islands, India, Japan, Africa, and South America, It is even stated that some of the Australian species are identical with those found in Europe.
Some of the fresh-water Polyzoa are extremely variable, and observers are by no means agreed in deciding whether certain well-known forms are to be regarded as varieties or as species. While certain genera, such as Cristatella and Lophopus, are comparatively constant in their form, Plumatella is excessively variable. Plumatella has a number of species greater than that of any other form, and the genus has a wider distribution than any other. This greater variation of species of the dominant genus is in complete accordance with the general law enunciated by Darwin that "wide-ranging, much diffused, and common species vary most."
While the ordinary forms of Plumatella consist of branching colonies, which are either completely adherent to their substratum, or grow in a more or less erect manner, another habit which is assumed by this genus is so different from the first that it has been considered to mark a distinct genus, Alcyonella. The Alcyonelloid form (Fig. 246, A) consists of closely packed tubes which stand more or less at right angles to their substratum, which they may cover with a dense mass an inch thick, and with a superficial area of several square inches. But in spite of this difference, it is possible that A. fungosa is only a variety of an ordinary Plumatella form. Whether this is so or not, a typical Plumatella may in places take on an Alcyonelloid habit; and parts of an Alcyonella may become so lax in growth as to resemble a Plumatella.