The nervous system consists of a central ganglion or brain, which is situated at the base of the tentacles on the side nearest the anus and gives out radiating nerves in all directions. Close to the brain and providing a communication between the cavity of the zooecium and the cavity in which the tentacles are contained (or, in the case of an expanded polyp, the external world) is a ciliated tube known as the "intertentacular organ." Apparently it acts as a passage through which the genital products are expelled; but contradictory statements have been made regarding it, and perhaps it is present only at certain seasons or in certain conditions of the polypide.
[Illustration: 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=oesophagus; 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 zooecium at the other, and by contracting pull the former back into the zooecium; (ii) the parieto-vaginal muscles, which connect the upper part of the invaginated portion of the zooecium with the main wall thereof; and (iii) the parietal muscles, which run round the inner wall of the zooecium and compress the zooecium 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 zooecium 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 zooecium 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.