INTRODUCTION TO PART III.
I.
Status and Structure of the Polyzoa.
The Polyzoa constitute a class in the third great division of the animal kingdom, the so-called Triploblastea. In this division are included also the worms, molluscs, insects, crustacea, spiders, vertebrates, etc.; for heterogeneous as its elements appear, all these animals may be considered to have essential features in common, in particular a body consisting primarily of three cellular layers. Most of them also possess a body cavity distinct from the alimentary canal. Some authors regard the position of the polyzoa as near that of the higher worms, but the group is an isolated one.
In considering the anatomy of simple forms of animal life such as the sponges it is necessary to pay attention mainly to individual cells, but in discussing more complicated forms our notice is first attracted to tissues and organs, for the cells of which these tissues and organs are composed have each a definite position, a definite structure, and a definite function. The most characteristic feature of the polyzoa, considered from this point of view, is the fact that most of their organs fall into one of two categories and are connected either with what is called the "zoœcium" or with what is known as the "polypide." The zoœcium is a cage in which the polypide is enclosed, but it is a living cage, differing from the shell of a snail or the tubes in which many worms encase themselves in being part of the animal itself. The polypide consists mainly of the organs connected directly and indirectly with nutrition and of part of the muscular system; its name is derived from the fact that it bears a superficial resemblance to a polyp such as Hydra.
The shape and structure of the zoœcium differs greatly in different groups of polyzoa. In its simplest form it is merely a cylindrical tube of living matter which secretes an outer horny or gelatinous covering. It is open at the end furthest from its base, at which it is attached either to another zoœcium or to some kind of supporting structure. Certain parts of the polypide can always be extruded from the aperture, which is known technically as the "orifice," or withdrawn through it into the zoœcium. When the polypide is retracted it draws in with it a portion of the zoœcium. The dead outer layer or ectocyst lines part of the portion thus invaginated and forms the walls of a cavity within the orifice. The base of this cavity consists in many forms of a transverse partition pierced in the middle by a circular hole and known as the "diaphragm." The diaphragm, however, does not constitute the limit of the invaginated portion of the zoœcium, for the living inner wall or endocyst is dragged in still further and forms a sheath round the retracted tentacles. When the tentacles are protruded they emerge through the hole in the diaphragm, carrying with them their sheath of endocyst. The invagination above the diaphragm, consisting of both endocyst and ectocyst, is then everted.
The tentacles are a characteristic feature of the polypide. Together with the base to which they are attached they are known as the "lophophore"; they surround the mouth, usually in a circle. They differ widely from the tentacles of Hydra in both structure and function, although they too serve as organs for the capture of prey; they are not highly contractile and are not provided with nettle-cells but are covered with cilia, which are in constant motion. When extruded they form a conspicuous calix-like crown to the zoœcium, but in the retracted condition they are closely pressed together and lie parallel to one another. They are capable individually of motion in all directions but, although they usually move in concert, they cannot as a rule seize objects between them.
The mouth is a hole situated in the midst of the tentacles. It leads directly into a funnel-shaped œsophagus, the upper part of which is lined with cilia and is sometimes distinguished as the "pharynx," while the lower part, the œsophagus proper, is a thin-walled tube that connects the pharynx with the stomach, which it enters on the dorsal side. The stomach is a bulky organ that differs markedly in form and structure in different groups of polyzoa. It is lined internally with glandular cells and the inner wall is sometimes thrown into folds or "rugæ." The part with which the œsophagus communicates is known as the "cardiac" portion, while the part whence the intestine originates is called the "pylorus" or "pyloric" portion. The intestine commences on the ventral side opposite the entrance of the œsophagus and nearly on a level with it, the bulk of the stomach depending between the two tubes. This part of the stomach is often produced into a blind tube, the fundus or cæcum. The alimentary canal may therefore be described as distinctly Y-shaped. The proximal part of the intestine is in some forms lined with cilia, and the tube as a whole is usually divided into two parts—the intestine proper, which is nearest the stomach, and the rectum, which opens by the anus not far from the mouth.
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 zoœcium 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.
