Fig. 30.—Vertical section through a polypide of Alcyonidium with the polypide retracted (after Prouho).
A=orifice; B=contracted collar; C=diaphragm; D=parieto-vaginal muscles; E=tentacles; F=pharynx; G=œsophagus; H=stomach; J=intestine; K=rectum; L=intertentacular organ; M=retractor muscle; N=testes; O=ovary; P=funiculus; Q=parietal muscles; R=ectocyst; S=endocyst.
The muscular system is often of a complicated nature, but three sets of muscles may be distinguished as being of peculiar importance, viz., (i) the retractor muscles, which are fixed to the base of the lophophore at one end and to the base of the zoœcium at the other, and by contracting pull the former back into the zoœcium; (ii) the parieto-vaginal muscles, which connect the upper part of the invaginated portion of the zoœcium with the main wall thereof; and (iii) the parietal muscles, which run round the inner wall of the zoœcium and compress the zoœcium as a whole. The parietal muscles are not developed in the Phylactolæmata, the most highly specialized group of freshwater polyzoa.
The cavity between the polypide and the zoœcium contains a reticulate tissue of cells known as the "funicular" tissue, and this tissue is usually concentrated to form a hollow strand or strands ("funiculi") that connect the outer wall of the alimentary canal with the endocyst.
This rapid sketch of the general anatomy of a simple polyzoon will be the best understood by comparing it with fig. 30, which represents, in a somewhat diagrammatic fashion, a vertical section through a single zoœcium and polypide of the order Ctenostomata, to which some of the freshwater species belong. The polypide is represented in a retracted condition in which the Y-shaped disposition of the alimentary canal is somewhat obscured.
In the great majority of cases the polyzoa form permanent colonies or polyparia, each of which consists of a number of individual zoœcia and polypides connected together by threads of living tissue. These colonies are formed by budding, not by independent individuals becoming associated together. In a few cases compound colonies are formed owing to the fact that separate simple colonies congregate and secrete a common investment; but in these cases there is no organic connection between the constituent colonies. It is only in the small subclass Entoprocta, the polypides and zoœcia of which are not nearly so distinct from one another as they are in other polyzoa (the Ectoprocta), that mature solitary individuals occur.
As representatives of both subclasses of polyzoa and of more than one order of Ectoprocta occur in fresh water, I have prefaced my description of the Indian species with a synopsis of the more conspicuous characters of the different groups (pp. 183-186).
Capture and Digestion of Food: Elimination of Waste Products.
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.
Although the tentacles may be correctly described as organs used in capturing prey, they do not themselves seize it but waft it by means of the currents set up by their cilia to the mouth, into which it is swept by the currents produced by the cilia lining the pharynx. The tentacles are also able in some species to interlace themselves in order to prevent the escape of prey. Apparently they have the power of rejecting unsuitable food, for they may often be observed to bend backwards and forwards and thrust particles that have approached them away, and if the water contains anything of a noxious nature in solution the lophophore is immediately retracted, unless it has been completely paralysed. In the phylactolæmata the peculiar organ known as the epistome is capable of closing the mouth completely, and probably acts as an additional safeguard in preventing the ingestion of anything of an injurious nature.