Polyzoa

“ The number of species of Polyzoa is about fifty-four--belonging to twenty-four genera. Of these genera it is believed that four will be found to be new, or hitherto undescribed, and it has been deemed requisite to modify the characters of several others upon the more extended survey of species afforded mainly by the present collection. ”

“ The marine forms are vastly more numerous, and more easily found, since not only pieces of weed upon which they grow are to be found upon every beach, but whole masses of leaf-like colonies, forming what is known as horn-wrack, may be plentifully found. Instead of the tentacles armed with sting-cells, like the anemone’s, possessed by the Hydrozoa, the Polyzoa have arms clothed with active cilia, by which the food is swept into the mouth, passing on into the stomach, and then through the intestine to another opening. ”

“ The respiratory organs lie in front of the mouth, and are in the form of fleshy tentacles or spiral appendages. These animals are more nearly allied to the worms than to the molluscs.
POLYZOA.
Characters of Polyzoa.—
These are almost exclusively marine forms, and are important as fossils. They form colonies (polypary or zoarium) , and by their branching, foliaceous or tufty growth resemble sea-weeds. The cells in which the separate zoöids lived have peculiar characters of their own, which serve to distinguish the different genera. ”

“ Among the Polyzoa we meet with creatures that encase themselves in horny shells, or calcareous coatings, such as the Madrepores, which, like submarine masons, elaborate the carbonate of lime which the sea supplies them with, into shelly retreats; and the tubed Hydrioda, which construct winding galleries and convoluted tubes, from the mouths of which they protrude their fans and tentacles in search of prey. ”

“ The Polyzoa are chiefly marine, and the common "sea-mat," often erroneously treated as a sea-weed, is a well-known form. A species of another order often picked up on our coasts is the Sertularia, or Sea-Fir, composed of delicate branching stems of a horny-looking substance, which, under a pocket-lens, is found to contain an immense number of small cells inhabited by Polyps. It is instructive to compare the two and note how much more advanced in structure is the Polyzoon than the polyp. ”

“ The comparative study of the development does not support the hypothesis that the Polyzoa (q.v.) are comparable with Phoronis. In Pedicellina, the only Polyzoon in which the alimentary canal of the larva is known to become that of the first adult individual, the line between the mouth and anus is ventral in the larva; and since there is no reversal of the curvature of the digestive loop during the metamorphosis it must be regarded as ventral in the adult. ”

“ There can, however, be little doubt that some kind of excretion takes place in the Polyzoa; and in considering what organs could possibly perform this work, our attention is arrested by the alimentary canal. The digestive organs of the young bud are perfectly colourless. As growth proceeds, certain parts acquire a yellowish, and later a brown colour. The degeneration of the polypide is followed by the grouping of large numbers of the free cells of the body-cavity into a mass which closely surrounds the incipient "brown body." ”

“ The Bryozoaires, or Polyzoa, as British naturalists prefer to call them, form the boundary-line which divides the humble mollusc from the humbler zoophytes. In consequence of this intermediate organization, these creatures were long considered as polyps; but De Blainville, Milne Edwards, and Ehrenberg, almost simultaneously began to separate them from the molluscs, and form them into a separate group. ”

“ No longer considered as sea-weeds in virtue of their forms and modes of growth, Polyzoa are now shown, by examination of their economy, to belong to the animal kingdom. It is found, then, that the discovery of real relationships involves analysis. ”

“ In the vast majority of the polyzoa, marine as well as freshwater, movement is practically confined to the polypide, the external walls of the zoœcium being rigid, the zoœcia being closely linked together and the whole zoarium permanently fixed to some extraneous object. In a few freshwater species belonging to the genera Cristatella, Lophopus, Lophopodella and Pectinatella, the whole zoarium has the power of progression. ”

“ Development and Affinities.—Few parts of the history of the Polyzoa are more fascinating than that which deals with their development; and it is probable that no other is capable of giving so much insight into the affinities of the several groups to one another and to other groups of the animal kingdom. ”

“ This distribution of tentacles is not, however, without exception. Among the fresh-water Polyzoa there are some genera, as Plumatella and Crystatella, in which the arrangement of these parts is very decidedly bilateral. Some species of them show us such relations of the individuals to one another and to their surface of attachment, as give a clue to the modification; but in other species the meaning of this deviation from the radial type is not obvious. ”

“ The food of all polyzoa consists of minute living organisms, but its exact nature has been little studied as regards individual species and genera. In Victorella bengalensis it consists largely of diatoms, while the species of Hislopia and Arachnoidea possess an alimentary canal modified for the purpose of retaining flagellate organisms until they become encysted. Similar organisms form a large part of the food of the phylactolæmata. ”

“ The Polyzoa are none the less a most attractive group. Let any one examine a dry piece of a brown paper-like substance (Fig. 232, A) , which may be found thrown up on the beach on many parts of our coasts. Of this species (Flustra foliacea) , the {466}so-called "sea-mat," an old writer says: "For curiosity and beauty, I have not, among all the plants or vegetables I have yet observed, seen any one comparable to this seaweed." ”

“ The true Zoophytes have all, more or less, the plant-like form, and they readily separate into two great classes, namely, the Anthozoa, or flower-life, and the Polyzoa, or many-life, in which the individuals are associated together in numbers. ”

“ Still stranger seems the superfluous exercise of power on examining the carpet of living forms at the bottom of the ocean. Not dwelling on the immense variety of creatures unlike in type which live miles below the surface in absolute darkness, it will suffice to instance the Polyzoa alone: low types of animals so small that a thousand of them would not cover a square inch, and on which, nevertheless, there has been, according to the view we are considering, an exercise of creative skill such that by small variations of structure more than 350 species have been produced! ”

“ Among Polyzoa the animal’s body is coated with a membraneous covering, like that of the Tunicated Mollusca, but which is a continuation of the edge of the cell, which doubles back upon the body in such a manner that when the animal protrudes from its cell it pushes out the flexible membrane just as one would turn inside out the finger of a glove. This oneness of cell and polype is a distinctive character of the group. ”

“ The avicularia of some of the zooecia have been omitted in B.
General Characters.—The Polyzoa are colonies, leaf-like or tree-like in form, and often strongly resembling seaweeds, or forming encrustations on the surface of stones and water-plants, or taking on other shapes. The units of the colony are complete individuals (Fig. 234) . The zooecium or body-wall encloses a body-cavity, in which lies a digestive canal, with which are closely connected the central nervous system and the retractile, ciliated tentacles. The structures other than the zooecium constitute the "polypide." ”

“ You will often find other forms of polyzoa in clear ponds and mill-pools; sometimes you would suppose you were looking at a mass of sponge, as in the case of Alcyonella, or the creeping root of some weed, as in Plumatella and Fredericella; but when the sponge-like mass or rootlets are placed in water you will observe numbers of little animals to show their heads and tentacles above the mass or from the little holes in the creeping rootlets. ”

“ The White or Coralline Crag is generally of a pale buff colour, and is in places almost entirely composed of the remains of Polyzoa. These (formerly called Corallines, whence the name Coralline Crag) are beautiful objects for a low-power microscope, or pocket lens, and are easily mounted in deep cells on slides. The bits of shell and sand that stick to them should be carefully removed with the point of a needle. A very large number of shells occur in this crag: of bivalves, the Pecten is very abundant, and its valves are frequently thickly grown over with Polyzoa ”

“ Many of the marine forms have a very wide distribution, Mr. Hincks having described several species as occurring from Norway to New Zealand. We are glad to see that Mr. Harmer still describes Mr. Hincks's 'History of the British Marine Polyzoa' as "invaluable," and his excellent contribution will, we feel sure, if studied, lead many more zoologists to study these somewhat neglected creatures, who are undoubtedly as interesting as "birds, beasts, or fishes." ”

“ The Polyzoa include freshwater as well as marine forms. They have a free-swimming larva, which becomes fixed after a time, and gives rise to the adult Colonial forms. The zooids of the latter have each an independent head with a crown of tentacles, called the Lophophore (Crest-carrier) ; but the fixed ends of their bodies communicate with one another. The hard covering of the colony, which retains its form after the animal is dead, is a kind of hardened skin: the apparent "cells" are the openings through which the individual zooids protrude themselves. ”

“ This sand, however, should be taken from the same bed in which the fossils occur, so as to get no admixture of the smaller shells from another formation or deposit; for although we may not wish to examine the finer material ourselves, it will yield in many cases a rich harvest to our microscopical friends, such residues containing microzoa, as shells of foraminifera, polyzoa and carapaces of the ostracoda. ”

“ The forms of animal life which are now united in an independent class, under the name Polyzoa, so nearly resemble the Hydroid Zoophytes in general form and appearance that a casual observer may suppose them to be nearly identical. ”

“ The order, Polyzoa infundibulata, is divided into three suborders, coinciding very nearly with the Tubuliporina, Celleporina, and Vesicularina of the work above referred to, but as the characters of these suborders are derived from the conformation of the opening of the cell, I have thought it more convenient to name them accordingly. The first suborder, having a round, simple opening to the cell, is here termed the Cyclostomata ”

“ Polyzoa are very abundant in the Cainozoic beds of Australia, New Zealand, and elsewhere (Fig. 25) . In the Mesozoic series, on the other hand, they are not so well represented; but in Europe and North America they play an important part in forming the Cretaceous and some Jurassic strata by the abundance of their remains.
WORMS (VERMES) .—The hard, calcareous tubes of Sea-worms, the Polychaeta (“many bristles”) are often found in fossiliferous deposits, and sometimes form large masses, due to their gregarious habits of life ”

“ No group of animals offers greater difficulty than the sponges, hydroids, and polyzoa (and especially the freshwater representatives of these three groups) as regards the question "What is a species?" ”

“ Of the organisms resulting from the aggregation of such creatures, we see that while some, as the Foraminifera, assume a certain definiteness of form, in their shells at least, others, as the Sponges, are very irregular. The Zoophytes and the Polyzoa are compound organisms, most of which have a mode of growth not more determinate than that of plants. But among the higher animals, we find not only that the mature shape of each species is very definite, but that the individuals of each species differ little in size. ”

“ In the avicularia and vibracula of the Polyzoa we have organs widely different in appearance developed from the same source; and with the vibracula we can understand how the successive gradations might have been of service. With the pollinia of orchids, the threads which originally served to tie together the pollen-grains, can be traced cohering into caudicles ”

“ It appears again on this new level in the aggregation of hydroid polyps, of polyzoa, of ascidians, and especially in the beautiful floating Siphonophora, in which the polyp-like units (themselves historically aggregates of cells) have become so subordinate in relation to the whole that they can often scarcely be recognized as individuals, and the individuality of the aggregate is much more marked than that of its components. It appears in a new way in the Termites and in the social Hymenoptera—ants, bees, and wasps. ”

“ The common Sea-anemones are but occasionally observed to undergo self-division: multiplication by budding being in some cases largely followed, but their numbers are not rapidly increased by either process. A higher class of secondary aggregates exemplifies the same general truth with a difference. In the smaller members the agamogenesis is incomplete, and in the larger it disappears. The gemmation of the minute Polyzoa, though it does not end in the separation of the young individuals, habitually goes to the extent of producing families of partially independent individuals ”