Lophopus crystallinus ([Plate IV]., No. 98) displays beautiful plumes of tentacles arranged in a double horseshoe-shaped series. When first observed these polyps resemble in many respects masses of the water snail ova, for which they are often mistaken. On placing these jelly-like masses into a glass trough with some of the clear water taken from the stream in which they are found, delicate tubes are seen to cautiously protrude, and the beautiful fringes of cilia are quickly brought into play. The organisation of L. crystallinus is simple, although it is provided with organs of digestion, circulation, respiration, and generation. The nervous[70] and muscular systems are well developed. This polyp increases both by budding and by ova, both of which conditions are shown in [Plate IV]., No. 98. The ova are enclosed in the transparent case of the parent. In Lophopus and some other fresh-water genera, Cristatella, Plumatella, and Alcyonella, the neural margin of the Lophopore is extended into two triangular arms, giving it the appearance of a deep crescent.
Another family presents a contrast: there is no lid to the mouth, and the tentacles are arranged in a circle on a disc. An important rise in organisation is found in the Gymnolæmata, especially in the lip-mouthed forms; the individuals belonging to this order vary in structure and fulfil different physiological functions. There are structures known as zoæcia, stolons, avicularia, vibracula, and ovicells, some of which are merely modified individuals. The zoæcia are the normal individuals of the colony, fully developed for most of the functions of life; the stolons have a much humbler function, but are indispensable—they are the root-like outgrowths of the stock, and serve for attaching the colony to foreign objects. The most remarkable are those known as avicularia, so called because they resemble the head of a bird. This process acts as a pair of forceps, the large upper blade of which is very like the skull and upper jaw of a bird, and the smaller lower blade (like the lower jaw) constantly opens and shuts by means of a complicated arrangement of muscles (shown in [Fig. 361]). These avicularia are movably attached by short muscles to the neck, and are found near the entrance to a zoæcium. They turn from side to side, snapping in all directions, catching at every particle of food that may come near; at length the morsel is drawn into the mouth by the cilia on the tentacles. From this very peculiar structure the Chilostomata were originally named bird’s-head corallines, then specifically shepherd’s-purse corallines, Notamia bursaria. Equally interesting, again, are the vibracula, long thread-like structures, attached by short footstalks. These keep up a constant whip-like motion, the object of which is not quite clear. The ovicells, or egg receptacles, are found at the lower ends of the zoæcia in the form of shields, helmets, or vesicles. In [Plate IV]., Nos. 95 and 96, a front and edge view of the statoblast is shown highly magnified.
Fig. 361.
1. Notamia bursaria, shepherd’s-purse Bryozoa; 2. Polyp magnified and withdrawn into its cell; 3. Portion of a colony of Hydroid polyps.
Another sub-order consists of the Cyclostomata, or round-mouthed Bryozoans, of which the Tubulipora is the typical form. The stocks are cup-shaped incrustations, the individuals radiating outwards, as in [Plate IV]., No. 92. Tubularia dumortierii is a very interesting form, the germinal bodies, statoblasts, being formed as cell masses on the strand, or funiculus, which also maintains the stomach in its place. They are round or oval in shape, and brown or yellow in colour, and consist of two valves fitted one upon the other like watch glasses, as shown in No. 96. A number of other statoblasts are shown, Nos. 97, 98, and 99. The edge running round No. 95 is seen to have barbed tips; the ring itself contains small air chambers, and is termed the swimming belt. It is, in fact, a perfect hydrostatic apparatus, giving support to the winter buds or statoblasts on the surface of the water. The barbed hooks apparently act as anchors, and by their means they catch on at points suitable for their development during the coming spring. As soon as the time comes, the two halves split apart and the germinal mass emerges forth. Out of these winter buds and statoblasts asexually produced individuals arise, which reproduce themselves sexually, their descendants again yielding winter germs. In short, an alternation of generations is a continually recurring process.
Fig. 362.—Lingula pyramidata.
Brachiopoda.—Here again we have to do with an enigmatical class of arm-footed animals, of which the Lamp-shells may be regarded as typical. These have remained unaltered from the earliest geological epochs. Brachiopods are divided into two orders: those having shells without hinges, and those with shells hinged together. On the whole they possess less interest for the microscopists than many other animals, except in their earliest developmental stages of existence.
One of the most interesting of the hinge-class group, living chiefly near the shores of the warmer seas, is the Lingulidæ. The valves are almost exactly similar, but are not hinged together, and have no processes for the support of the thick fleshy spiral arms of the animals. In L. pyramidata, found around the Philippine Islands ([Fig. 362]), the stalk is nine times longer than the body. The animal does not attach itself by this, but moves about like a worm, making tubes out of sand, into which it can withdraw itself and disappear. The cilia at the mantle edge form a fine sieve, thus preventing foreign particles from entering the gills. Its internal structure possesses points of interest, and the parasitic growths covering the cartilaginous structure, miscalled a shell, are curious, and excite the attention of the naturalist.