Respiration is provided for in the Bryozoaires by the ciliate appendages which surround the mouth; they are at once tentacula and branchiæ. The animal presents no other trace of organs of the senses. A small ganglion and a few fillets constitute all of the nervous system which can be traced; neither heart nor blood-vessels have been found.
The egg, in the case of the Bryozoaires, gives birth to a young animal covered with hairs on its surface; it swims about freely until it has chosen a convenient place in which it can establish the new colony which it is to originate. But this choice is not made for itself alone; the young animal encloses under its hairy envelope two new individuals, which, young as they are, have already the appearance of adult Bryozoaires. At first, these only increase the number of the colony by budding, but in a short time they produce eggs.
From these remarks it will be seen that the animals of the Bryozoaires are more complex in their form and functions than those of the coral, and the study of their anatomy confirms this conclusion. In their case the digestive organs are no longer a simple sac with a single orifice; there is a mouth, a pharynx, a gullet, a gizzard, a membranous stomach and intestines, with a special opening. We have descriptions of some species in which the gizzard seems to be provided with a certain number of interior teeth forming a wonderful pavement—a living mill for the purpose of grinding the food before it enters into the second stomach. The organization of this small creature reveals to our eyes a wonderful amount of combination—of admirable art immeasurably surpassing all that the most perfect human industry and human genius can accomplish.
Fig. 121. Plumatella cristallina magnified (after Roesel).
After this general view of the organization of the group, we shall proceed to introduce the reader to some of their more characteristic species.
Under the leaves of water-lilies (Nymphea), pond-weed (Potamogeton), or upon floating fragments of submerged wood, are generally to be found certain Bryozoaires, animals described by Trembley under the name of plumed polyps. These are Plumatellæ (Fig. 121). These little diaphanous creatures constitute colonies which under the microscope resemble small branching shrubs; they consist of small slender tubes grafted one to the other, and having from forty to sixty retractile tentacula, which expand like the petals of a flower; they are furnished with vibratile cilia, the movements of which serve the purpose of leading food into the mouth.
Another genus, which is found in ponds in France, and which is also found in fresh water in Britain, is the Cristatella of Cuvier. "Perfect specimens of C. mucedo occur from six lines to twenty-four in length by two or three in breadth," says Sir J. G. Dalyel, "of a flattened figure, fine translucent green colour, and fleshy consistence. Some of the shorter tend to an elliptical form, but those of larger dimensions are linear, with parallel sides, and curved extremities. The middle of the upper and the whole of the under surface are smooth, the former somewhat convex, occasioned by a border of seventy or eighty, even up to three hundred and fifty, individual polypi, dispersed in a triple row, their number depending entirely on the size of the specimen. Each of these numerous polypi, though an integral portion of the common mass, is a distinct animal, endowed with separate action and sensation. The body rising about a line above a tubular fleshy stem, is crowned by a head, which may be circumscribed by a circle as much in diameter, of a horse-shoe shape, and bordered by a hundred tentacula. Towards one side, the mouth, of singular mechanism, seems to have projecting lips and to open as a valve, which folds up within, conveying the particles which are absorbed to the wide orifice of an intestinal organ, which descends, perhaps, in a convolution below; and returns again, terminating in an excretory canal under the site of the tentacula."
Fig. 122. Cristatella mucedo (Cuvier).