"To give, however, more actuality to our ideal Polyzoon, we may bear in mind that the immediately investing sac has the power, in almost every case, of secreting from its external surface a secondary investment, of very various constitution in the different groups; and we may, moreover, conceive of the entire animal with its digestive tube, tentacula, ganglion, muscles, generative organs, circumambient fluid, and investing sacs, repeating itself by gemmation, and thus producing one or more precisely similar systems holding a definite position relatively to one another, while all continue organically united, and we shall then have the actual condition presented by the Polyzoa in their fully-developed state."
The vast majority of the Polyzoa are fixed, but this is not universally the case. Thus the singular fresh-water Cristatella is free and locomotive, creeping about by means of a flattened discoid base, not unlike the foot of the Gasteropoda; and the polyzoary seems to have been unattached in a few other forms (Selenaria, Cupularia, etc.)
The two fundamental structures of the "coenoecium" of a Polyzoon - viz., the immediately investing sac, and its secondary investment - are sometimes termed the "endoderm " and "ectoderm;" but as these terms are employed in describing the Hydrozoa, it is better to make use of the terms "endocyst" and "ectocyst," proposed by Dr Allman.
The "ectocyst," or external investment of the coenoecium, is usually a brown, pergamentaceous, probably chitinous, but often highly calcareous, membrane; and it is by the ectocyst that the "cells" are formed. In Cristatella, alone of the Polyzoa, there is no ectocyst; and in Lophopus (fig. 199, 3) and in the curious Pectinatella the ectocyst is gelatinous in its consistence. In many cases the ectocyst is provided with singular appendages, supposed to be weapons of offence and defence, or organs of prehension, termed "avicularia" (fig. 198, 3) and "vibracula." The avicularia, or "bird's-head processes," differ a good deal in shape, but consist essentially of " a movable mandible and a cup furnished with a horny beak, with which the point of the mandible is capable of being brought into apposition" (Busk). In shape the avicularia often closely resemble the head of a bird, and they are in many respects comparable with the "pedicellariae" of the Echinodermata, keeping up a constant snapping movement, which continues long after the death of the general colony. In the "vibracula," the place of the mandible of the avicu-larium is taken by a bristle, or seta, which is capable of extensive movement. In many cases the cells are also furnished with globular sacs or pouches ("ovicells" or "oocysts"), appended to them, and serving as marsupial pouches for the ova. Ovicells are only known in the marine Polyzoa, and the ova are liberated by their ultimate rupture.
It is generally believed that the avicularia, vibracula, and ovicells are really undeveloped polypides or modified zooids. Good authorities also believe that the "cells" or "zooecia" themselves are not to be regarded as mere skeletal structures, but that they have a life independent of that of their contained polypides, and that they can continue to live and produce new polypides after the death of the latter. They are regarded, in fact, as separate zooids.
The endocyst is always soft, contractile, and membranous; and, according to Sars, is wanting in Rhabdopleura. It lines the interior of the cells formed by the ectocyst, and is reflected backwards at the mouth of the cell, so as to be invaginated, or inverted into itself; and it finally terminates by being attached to the base of the circlet of tentacles. This invagination of the endocyst is more or less permanently present in all the fresh-water Polyzoa. The epithelium lining the inner surface of the endocyst is furnished with vibratile cilia.
The mouth of each polypide is surrounded by a crown of tubular, non-retractile tentacles, which have their sides ciliated, and are arranged sometimes in a circle and sometimes in a crescent. In the fresh-water Polyzoa the tentacles are united towards their bases by a funnel-shaped membrane, known as the "calyx." The tentacles are borne on a kind of disc, or stage, which is termed by Professor Allman the "lophophore." In the majority of Polyzoa - including almost all the marine species - the lophophore is circular (fig. 199, 2); but in most of the fresh-water forms it has its neural side extended into two long arms, so that the entire lophophore becomes crescen-tic or "horse-shoe-shaped " (fig. 199, 3); hence this section is sometimes collectively termed the "Hippocrepian" Polyzoa, In all, or almost all, the Polyzoa in which this crescentic condition of the lophophore exists, there is also a singular valvelike organ which, springing from the anal side of the lophophore, arches over the mouth, and is termed the "epistome." The only marine forms in which the lophophore is bilateral are Pedicellina and Rhabdopleura; the only fresh-water species in which the lophophore is orbicular are Paludicella and Urna-tella.
Fig. 199. - 1. Fragment of Flustra truncata, one of the Sea-mats, natural size. 2. A single polypide of Valkeria, magnified, showing the orbicular crown of tentacles. 3. A polypide of Lophopus crystallinus, a fresh-water Polyzoon, highly magnified, showing the horse-shoe-shaped crown of tentacles. a Tentacular crown; b Gullet; c Stomach ; d Intestine; e Anus; g Gizzard ; k Endocyst; l Ectocyst; f Funiculus.