The Metazoa fall into two divergent branches of very different importance. The group Parazoa includes only the sponges (q.v.), whilst the equivalent group of Enterozoa includes the remainder of the animal kingdom.
A Parazoan is an animal which, when adult, is permanently fixed to a substratum. Its body is composed of tissues which provide it with an external surface, and surround a cavity usually very complex and often irregular in shape, through which a cur rent of water is caused to pass by the activity of the flagella borne by the cells which surround it. The cavity of a sponge, the canal system, is always in communication with the sea by inhalant and exhalant openings; the latter are oscula, the former differ in their morphological nature in different sponges.
Some part of the canal system is always lined by an epithelium composed of cells, choanocytes, which possess a single flagellum, whose base arises within a cup formed by an extremely delicate protoplasmic membrane which projects from the free surface of the cell. This collar can be retracted, and serves in some way, not yet understood, to affect the current of water caused by the lashing of the flagellum.
These collar cells are found only in Parazoa amongst the Meta zoa, but identical cells form the bodies of a group of Protozoa, the Choanoflagellata. The collar cells not only serve to main tain the current of water, which, passing continuously through a sponge, brings to it its food and oxygen, but are the actual nutritive mechanism, engulfing food particles and digesting them. From them also arise the gametes. The remainder of a sponge consists of an external epithelium, protective in nature and an intervening mesoglosa containing cells of many different types and skeletal elements of varied nature.
During the development of a sponge the cells which, in the adult, are collar cells, form an external layer and serve for the locomotion of the free swimming embryo, whilst all the other cells of the body, including those which form the external sur face of the adult, lie internal to them, often as a compact mass. (See INVERTEBRATE EMBRYOLOGY.) The Enterozoa form a complete contrast to the Parazoa. They present many grades of structural complexity, of which the lowest is that presented by the members of the phylum Coelenterata (q.v.), from which all the others have ultimately been derived.
The primitive Enterozoan has a body consisting of a sac, whose cavity, the archenteron, or primitive gut, opens to the exterior by a single aperture, through which food enters and the faeces are expelled; it thus combines the functions, and, indeed, probably gave origin to both mouth and anus.
The body-wall of a primitive Enterozoan consisted of two epithelia placed back to back, their cells being attached to a base ment membrane common to both.
The epithelium which lines the single cavity is the endoderm; its cells are primitively concerned only with food, carrying out the processes of digestion, assimilation and food transport, and also giving rise to the germ cells.
The outer layer, the ectoderm, is that part of the animal which is brought into direct contact with the environment ; it is respon sible for protection, locomotion, the perception and capture of food, respiration and the excretion of nitrogenous waste products. In it arises the nervous system and sense organs which are necessary for the adjustment of the animal's behaviour to ex ternal circumstances. During the development of such an animal, the endoderm arises from cells which at no time lie external to those which compose the ectoderm.
The complexity of structure which may be built up on so simple a plan as that of a Coelenterate is very considerable, but can not approach that which is, in fact, found amongst Metazoa. The first step in further evolution was the addition of a further layer of cells, the mesoderm, not necessarily epithelian in ar rangement, between the ectoderm and endoderm.
In the majority of Coelenterates the basement membrane, which lies between these two primitive epithelia, becomes greatly thick ened as a mesoglosa, into which cells wander chiefly, if not ex clusively, from the ectoderm. In one living group of animals (the Ctenophora) this migration takes place precociously, so that a special mass of cells, derived from the embryonic ecto derm, is set apart as a mesoderm. From this the whole muscu lature of the adult arises.