Nervous system consisting of a single ganglion, or of a principal pair with accessory ganglia; no distinct organ of the circulation, or an imperfect heart.
The members of this class are defined as follows: - "Alimentary canal suspended in a double-walled sac, from which it may be partially protruded by a process of evagination, and into which it may be again retracted by invagination. Mouth surrounded by a circle or crescent of hollow, ciliated tentacles; animals always forming composite colonies" (Allman).
All the Polyzoa live in an associated form in colonies or "polyzoaria," which are sometimes foliaceous (fig. 197), sometimes branched and plant-like, sometimes encrusting, and very rarely are free. Each "polyzoarium" consists of an assemblage of distinct but similar zooids arising by continuous gemmation from a single primordial individual. The colonies thus produced are in very many respects closely similar to those of many of the Hydroid polypes, with which, indeed, the Polyzoa were for a long time classed. The "polyzoarium," however, of a Polyzoon differs from the polypidom of a composite Hydroid in the general fact that the separate cells of the former do not communicate with one another otherwise than by the continuity of the external integument; whereas the zooids of the latter are united by an organic connecting medium, or "coenosarc," from which they take their origin.
In one group of the Polyzoa - viz., the Ctenostomata (including Vesicularia and its allies), the cells arise from a common stalk, and are thus placed in communication with one another; but this hardly affects the general value of the distinction above spoken of.
The homomorphism, however, which subsists between the
Fig. 197. - Flustra foliacea, one of the Sea-mats. a Portion of the colony, natural size ; b A fragment magnified, to show the cells in which the separate polypides are contained.
Polyzoa and the Hydroida, is shown most decisively not to be a true affinity, when the structure of the individual zooids is examined. The polypite of a Hydroid Zoophyte, as we have already seen, possesses no alimentary canal distinct from the general cavity of the body; there are no traces of a nervous system, and the reproductive organs are in the form of external processes of the body-wall. In the zooid of all the Polyzoa (fig. 198, 3), on the other hand, there is a distinct alimentary canal, completely shut off from the somatic cavity; a nervous system is present, and the reproductive organs are contained within the body.
In the Polyzoa, the entire colony - or its entire dermal system - is called the "polyzoarium" or "coenoecium;" the separate zooids are called "polypides;" and the little chambers in which each is contained are called the "cells," or "zooecia."
It will be seen, therefore, that the term polypite is restricted to the zooid of a compound Hydrozoon, or to the entire hydro-soma of a simple member of the class. The term polype is applied to a simple Actinozoon, or to the zooids of a compound actinosoma. Lastly, the term polypide is exclusively employed to designate the zooid of one of the Polyzoa.
The construction of a typical polypide of a Polyzoon is thus described by Professor Allman (fig. 198, 2):
"Let us imagine an alimentary canal, consisting of oesophagus, stomach, and intestine, to be furnished at its origin with long ciliated tentacula, and to have a single nervous ganglion placed upon one side of the oesophagus. Let us now suppose this canal to be bent back upon itself towards the side of the ganglion, so as to approximate the termination to the origin. Let us further imagine the digestive tube thus constituted to be suspended in a fluid contained in a membranous sac with two openings, one for the mouth and the other for the vent, the tentacula alone being external to the sac. Let us still further suppose the alimentary tube, by means of a system of muscles, to admit of being retracted or protruded according to the will of the animal; the retraction being accompanied by an invagination of the sac, so as partially or entirely to include the oral tentacles within it; and if to these characters we add the presence of true sexual organs in the form of ovary and testis, occupying some portion of the interior of the sac, and the negative character of the absence of all vestige of a heart, we shall have, perhaps, as correct an idea - apart from all considerations of homology or derivation from an archetype - as can be conveyed of the essential structure of a Polyzoon in its simplest and most generalised condition.
Fig. 198. - Morphology of Polyzoa. 1. Portion of the coenoecium of Flustra truncata, magnified. 2. Diagram of a Polyzoon (after Allman): a Region of the mouth surrounded by tentacles ; b Alimentary canal; c Anus ; d Nervous ganglion ; e Investing sac (ectocyst); f Testis; f Ovary; g Retractor muscle. 3. Bird's-head process, or "avicularium," of a Polyzoon.