POLYZOA.
(*) An asterisk against names of species denotes that specimens of these species are in the upright part of Case A and preserved in spirit.
Upright Table-Cases A and B, at south end of Shell Gallery.
From a casual glance at the contents of these cases, it might be supposed that many of the specimens exhibited therein were seaweeds; but a closer inspection, especially with a lens, will reveal structure of a kind not to be found in any plant.
Let us select for examination Flustra foliacea, the Broad-leaved Hornwrack or Sea-Mat (Fig. 1), (Case A 1), commonly to be found among heaps of seaweed cast up on sandy shores round our coasts.
Fig. 1.
Flustra foliacea. A, natural size; B′, portion magnified in B; B, magnified 30 diameters.
a, avicularium; o, ovicell.
[‘The Cambridge Natural History.’]
The brown horny fronds, which vary in width, branch upwards from a narrow flat stem attached at its base to stones and shells. Both surfaces of the fronds show a fine network pattern formed by the edges of little oblong boxes or cells termed zoœcia,[[10]] arranged in longitudinal parallel rows and forming a double layer back to back. The cells are broad and rounded above, narrow and truncate below, and each is roofed in by a transparent membrane with a semicircular lid or operculum situated near the upper end; four short stout spines spring from the margin in this neighbourhood. When the surface of a living frond is examined in sea-water, here and there a bundle of tentacles may be observed pushing up a lid, slowly emerging and expanding into a bell-shaped coronet; on the least alarm the tentacles are rapidly withdrawn into the cell and the lid shut. The flexible protrusible region of the cell is termed the tentacle sheath. The relation of the cell to the tentacle sheath (Figs. 2, 3) may be roughly compared to a glove finger, stiff below, but flexible at the end, and surmounted by a crown of bristles; on pulling down the glove-finger tip, the tentacles will also be drawn in, and will lie in a sheath formed by the invaginated portion of glove finger. The lid which closes over the tentacle sheath is only found in the Sub-order Chilostomata to which Flustra belongs. The area of the tentacle sheath whence the tentacles arise is termed the lophophore.[[11]]
Figs. 2, 3, diagrams representing polypide in cell. Fig. 2, tentacle-sheath protruded. Fig. 3, ditto, retracted; a, tentacles; b, tentacle-sheath; c, mouth; d, gullet; e, stomach; f, vent; g, retractor muscle; h, funiculus; l, ovary; k, testis; l, lid or operculum; nerve ganglion is between mouth and vent. Fig. 4, polypide extracted from cell; d, pharynx; e, stomach; f, anus (after Van Beneden). Fig. 5, section (partly diagrammatic) of frond of Flustra, showing cells back to back.
The mouth is situated in the centre of the lophophore, surrounded by the circle of tentacles; and the latter, by the action of their cilia, set up currents which convey food to the mouth.
The mouth leads into a pharynx and gullet, the latter opening into a stomach, whence the intestine ascends to terminate in the vent opening below and outside the circle of tentacles; the intestines, in fact, form a U-shaped tube (Figs. 4, 5) suspended in the body-cavity in the interior of the cell. A cord, the funiculus, passes from the stomach to the base of the body-cavity. A small nerve ganglion is situated within the upper part of the loop of intestine.
The tentacles, intestines, and other organs constitute the “polypide,” the cell being simply the protective house formed by the latter.
The body-cavity, which contains fluid, is in direct communication with the interior of the tentacles, which are hollow, and which act as respiratory organs by bringing the fluids of the body-cavity in proximity to the water. In Flustra the body-cavities of the cells are shut off from each other, but pores and sieves in the partition walls allow of the junction of the inner linings of these cavities. The male and female reproductive elements are formed in the body-cavity. The egg develops in a helmet-shaped brood-pouch, the ovicell, situated at the summit of the cell and almost immersed in the cell above. The ciliated embryo swims about for a few hours and settles down to form the first polypide and cell; from the latter there arise buds which remain attached, and produce other buds, till a colony like that of Flustra results.
Among the ordinary cells are certain smaller cells (Fig. 1, a) slightly raised above the general level, different in shape from the ordinary kind and with thicker lids. These peculiar cells are termed avicularia, and chiefly contain muscles for opening and shutting the lid. They arise by modification of the ordinary cells, whereby all the organs of the polypide have become atrophied except the muscles. The Polyzoa[[12]] were so named by Vaughan Thompson, who, in 1820, discovered that certain plant-like animals, which had previously been classed with the zoophytes, possessed a much higher organisation, in that the intestine was separate from the body-cavity and not continuous with it as in Sea-Firs, Sea-Anemones, and Corals. In 1834, Ehrenberg named the group Bryozoa[[13]] or Moss Animals.
With the exception of one genus (Loxosoma), all Polyzoa form colonies, which arise by the continual budding of the cells, the buds remaining attached to the parent cells. The colonies vary endlessly in form and habit, occurring as crusts on rocks, etc., masses, broad fronds, branching tree-like growths, bushy tufts, etc.
The texture and consistency may be gelatinous, cartilaginous, horny and flexible, or stony.
The great majority of species are marine, but a considerable number inhabit fresh water. The Polyzoa are classified as follows:—
[[14]][[15]][[16]][[17]][[18]][[19]][[20]]
Sub-order 1.—Chilostomata.
Cases A and B 1.
The Chilostomata, which contain many more species than all the other groups put together, are divided into three sections:—A. Cellularina, in which the cells are more or less boat-shaped or cornucopia-shaped, and joined together to form flexible branching colonies; B. Flustrina, in which the cells are typically shaped like oblong boxes with membranous front walls; and C. Escharina, in which the whole front wall is calcified.
Case A 1.
Fig. 6.
A, Bugula turbinata, natural size, B, portion × 50.
a, avicularia; m, mouth; o, ovicell.
[‘The Cambridge Natural History.’]
Section A. Cellularina.—Bugula turbinata, or the Bird’s-head Coralline (Fig. 6) grows attached to rocks near low water mark in the form of spiral tufts about two inches in height, composed of narrow flat branches in which the cells are arranged from two to six abreast and all facing upwards. Each cell is boat-shaped and with nearly the whole front surface membranous; the globular bodies at the head of certain cells are the ovicells. Attached to the outer edge of each cell is a remarkable object resembling a bird’s head, and hence termed avicularium, seated on a short stalk. The head and beak contain powerful muscles for opening and shutting a horny lid or mandible hinged on below. In life, the avicularium sways to and fro on its stalk, with the lower “jaw” continually snapping up and down in the most ludicrous fashion. The beak is capable of seizing and holding quite large objects.
The function of these curious appendages is partly to warn off trespassers and partly to capture and retain small animals till decomposition has set in; in the latter case, the currents set up by the tentacles draw in the particles to the mouths of the polypides. The avicularia have arisen by modification of the ordinary cells, in which the muscles have developed at the expense of the degenerated polypides, the cells have become much smaller, of different shape, and separated out from the rest; the mandible represents the lid or operculum of the ordinary cell. The avicularia vary greatly in size and shape in the different genera; in Flustra, for instance, these organs closely resemble the ordinary cells.
Fig. 7.
Bugula bicornis. Cells magnified. (After Busk.)
Case A. Upright part.
In Bugula bicornis(*) (Fig. 7), from 1950 fathoms in the Southern Indian Ocean, each cell is provided with two avicularia with remarkably long stalks. The graceful vase-shaped Kinetoskias cyathus(*) (Fig. 8), one of the treasures of the “Challenger” Expedition, was dredged from 1525 fathoms off Cape St. Vincent. The stem, which tapers gradually upwards, rises from a tuft of root fibres. The cup is formed of slender branches supported at the base by a delicate membrane. The branches are composed of biserial rows of cells (Fig. 9) opening towards the interior of the cup. The avicularia are pear-shaped and pedunclate. Probably, in life, the cup is capable of being opened out to a considerable extent. Specimens of this species were also obtained from 2160 fathoms in the South Atlantic.
Fig. 9.
Kinetoskias cyathus. A branch magnified.
a, an avicularium. (After Busk.)
Case A 1.
Scrupocellaria reptans, or the Creeping Coralline (Fig. 10 A, B) forms branching colonies, creeping over rocks and seaweeds, and attached by horny fibres often provided with curved hooks. The branches are composed of cells arranged in a double row. Each cell has the membranous area of its front surface protected by a branched flattened spine or operculum, and is produced and narrowed below; at the upper outer margin is a minute triangular avicularium. At the base of the back surface is a small sack-shaped cell with a cleft at the upper end, in which a horny bristle is articulated. The little cell is termed a vibracular cell, and the bristle a vibraculum.[[21]] This organ has arisen by a further modification of an avicularium, whereby the horny lid of the latter has become a long bristle. The bristles by their motion keep off intruders, and possibly act as scavengers by sweeping the surface of the cells.
Fig. 8.
Kinetoskias cyathus. (From Voy. Challenger, Atlantic: Wyv. Thomson.)
Fig. 10.
Scrupocellaria reptans. A. Creeping over seaweed, natural size; B. Front surface, magnified.
a, branched spine covering front of membranous area; b, avicularium; c, vibraculum.
C. Back surface; a, vibracular cell; b, vibraculum.
In Caberia ellisii the vibracular cells are very large. The vibracula, which are long and serrated, have been observed to move in unison like a double row of oars.
Case A 1, 2.
Section B. Flustrina.—In this group the colonies form leafy lamellæ, crusts, etc., in which the individual cells are typically in the form of oblong boxes with their front walls wholly or partly membranous. Flustra foliacea has already been described. |Case A 1.| In Flustra carbasea the fronds are formed of only one layer of cells, and not of two layers back to back as in F. foliacea. The fine specimen of Flustra nobilis from S. Africa is so called from the large size of its long hexagonal cells which form a honeycomb pattern clearly visible to the naked eye.
Case A 1.
In Flustra cribriformis(*) (Fig. 11), from Torres Straits, the fenestrated frond forms a beautiful spiral. Flustra florea, from S. Australia, grows in the form of branching tufts of narrow spiral fronds. Electra pilosa [dry and spirit specimens exhibited] (Fig. 12) forms a delicate silvery lace-work, encrusting shells and seaweeds (especially red algæ) on almost every shore. The long horny spine at the base of the membranous area of each cell gives the crust a pilose appearance. In Electra verticillata from West Africa, the cells form an elegant branched colony, the branches being composed of regular verticils of cells.
Fig. 11.
Flustra cribriformis.
Fig. 12.
Electra pilosa. A, incrusting a seaweed, natural size; B, cells magnified; a, lid or operculum.
Case A 1.
Membranipora membranacea occurs in the form of horny incrustations on bladder-wrack, which, owing to their flexibility, are able to adapt themselves to the swaying of the fronds of the Fucus.
The Selenariidae (Case B 2) form free colonies, usually orbicular in shape, convex above and concave below. In Lunulites capulus alternating rows of cells and vibracula radiate from the centre of the colony.
Section C. Escharina.—In this group, the front walls of the cells are wholly calcareous. Many species form patches or crusts on shells etc., and hence the name of the section; other species, again, form stony tree-like growths, or thick plates. Frequently one and the same species occurs in the form of crusts or of erect lamellæ, the identity being recognised by the characters of the individual cells.
Often a large number of species may be found on one shell. Two good examples of this are exhibited in Case A 2.
Case A 2.
Lepralia pallasiana (Fig. 13) forms sub-circular vitreous patches on stones and shells; the cells are rather large, broadly oval, and with the front wall punctured with pores; the aperture is squarish and with a slight indentation on each side.
Fig. 13.
Lepralia pallasiana, incrusting a shell. A, natural size; B, cells magnified.
Lepralia foliacea forms a massive coral-like growth composed of thin contorted plates which fuse to form labyrinthine cavities, the plates being constructed of a double layer of cells back to back. A large specimen from the English Channel is exhibited in Case B, upright part. In Lepralia the orifice and lid of the cell have a straight lower margin, but one large group, Myriozoidæ, is characterised by having a notch in the lower margin of the orifice, (Fig. 14, Schizoporella unicornis).
Case B 1.
In many of the Escharina, the front wall of the cell is produced into a stout process or mucro at the lower margin of the orifice (genus Mucronella), or, again, a collar or tube grows up round the primary orifice, thus giving rise to a secondary orifice (Smittia, Porella, etc., Case B 1).
Fig. 14.
Schizoporella unicornis, magnified.
Fig. 15.
Retepora beaniana.
Case B 1.
In the Celleporidæ, (Case B 1) the cells are typically pitcher-shaped and arranged vertically, and tend to be heaped up from the overcrowding.
Cellepora pumicosa forms thick pumice-like masses composed of succeeding layers of cells. The Reteporidæ (Case B 1) form delicate stony networks. The reticulate fronds may be expanded out, or may form tubular or contorted growths (Fig. 15, Retepora beaniana). The beautiful Retepora phœnicea from Torres Straits is of a rich purple colour.
The Adeonidæ form thick fenestrated plates which unite to form cavernous masses usually attached to rocks by a thick jointed stem. Several very fine examples from Port Phillip, Victoria, are exhibited in the upright part of Case B.
Case A 2.
The Catenicellidæ are represented by a fine series of specimens from Australia. The colonies form dense clusters of finely beaded branches. The cells are arranged in single series, each cell being united to those above and below by a horny joint. The cells are usually urn-shaped with a triangular avicularium at each upper angle, and with the front surface variously sculptured with pores or bands (Fig. 16, Catenicella ventricosa).
Fig. 16.
Catenicella ventricosa. A, natural size; B, magnified. (After Busk.)