The presence of a complete digestive canal and the ciliation of the tentacles in Polyzoa are conspicuous differences between these animals and the Hydroids, with some of which the Polyzoa may have a marked external similarity.
The outermost[[505]] layer of the body-wall is known as the "ectocyst" (Fig. 234, e). This may be densely calcareous, in which case the dried Polyzoon differs little in appearance from the living animal with its tentacles retracted; or it may be partially calcified, or it may consist entirely of a flexible cuticle, as in Fig. 234. The ectocyst is prolonged through the orifice (o) as far as the diaphragm (d).
Forms with a calcareous ectocyst are commonly ornamented with ridges or other patterns, which are often of great beauty. The ectocyst in these cases is commonly interrupted at intervals by pores (Fig. 239, C), into which processes of the "endocyst"—the living, internal part of the body-wall—extend. These may appear as superficial pores, which apparently open to the exterior in the dried condition, or they may perforate the septa between adjacent individuals. This may be strikingly demonstrated by decalcifying a branch of Crisia (Fig. 237), in which the zooecia then appear connected by numerous strands of tissue. In many marine forms the communications between the individuals are in the form of small sieve-like plates known as "rosette-plates."
The endocyst may consist of definite layers of ectoderm (em) and mesoderm (mm), as in Fig. 234, but the mesoderm is commonly in the form of a loose network, some of which is attached to the body-wall, some to the alimentary canal, some forming connecting strands between these two layers, and other cells floating about freely in the body-cavity. These mesodermic structures are often spoken of as the "funicular tissue," since one or more strands of it commonly take on the form of a definite "funiculus" (f). This structure may bear the ovary (ov), while the testes (t) are found, commonly in the same zooecium, attached to various parts of the body-wall. The eggs and spermatozoa, when ripe, break off and float freely in the body-cavity.
The funicular tissue was at one time described as a "colonial nervous system." The idea expressed by this term must be considered erroneous from the fact that no nervous co-ordination of the individuals is known to exist, in the vast majority of cases. The actual nervous system consists of a ganglion (g) placed between the mouth and anus of each polypide, and lying in a small circular canal (not shown in Fig. 234) which immediately surrounds the oesophagus. This canal is developed in the bud as a part of the body-cavity, from which it becomes completely separated in marine forms. The Polyzoa have no vascular system.
Brown Bodies.—In the majority of cases, an extraordinary process of regeneration takes place periodically during the life of each zooecium. The tentacles, alimentary canal, and nervous system break down, and the tentacles cease to be capable of being protruded (Fig. 235, 1). The degenerating organs become compacted into a rounded mass (Fig. 235, 2 and 3, b.b), known from its colour as the "brown body." This structure may readily be seen in a large proportion of the zooecia of transparent species. In active parts of the colony the body-wall next develops an internal bud-like structure (Fig. 235, 1, b), which rapidly acquires the form of a new polypide (Fig. 235, 2 and 3). This takes the place originally occupied by the old polypide, while the latter may either remain in the zooecium in the permanent form of a "brown body," or pass to the exterior. In Flustra the young polypide-bud becomes connected with the "brown body" by a funiculus (Fig. 235, 1, 2). The apex of the blind pouch or "caecum" of the young stomach is guided by this strand to the "brown body," which it partially surrounds (3). The "brown body" then breaks up, and its fragments pass into the cavity of the stomach, from which they reach the exterior by means of the anus.
Fig. 235.—Flustra papyrea Pall. Naples. × 50. Illustrating the development of a new polypide after the formation of a "brown body." In 1, a, two masses formed from the alimentary canal; b, young polypide-bud; b.b, degenerating tentacles; c, connective tissue: 2, another zooecium, later stage; b.b, brown body; r.m, retractor muscles; s, stomach; t, tentacles of new polypide; t.s, tentacle-sheath: 3, the same zooecium, 191 hours later; letters as in 2. 1 and 2 are seen from the front, 3 from the back.[[506]]
There is some reason to believe[[507]] that these remarkable processes are connected with the removal of waste nitrogenous matters. The Marine Polyzoa are not known to be, in most cases, provided with definite excretory organs, although it is possible that the intertentacular organ (Fig. 234, i) described on p. [508] may in some cases perform excretory functions. There can, however, be little doubt that some kind of excretion takes place in the Polyzoa; and in considering what organs could possibly perform this work, our attention is arrested by the alimentary canal. The digestive organs of the young bud are perfectly colourless. As growth proceeds, certain parts acquire a yellowish, and later a brown colour. The degeneration of the polypide is followed by the grouping of large numbers of the free cells of the body-cavity into a mass which closely surrounds the incipient "brown body." Under their action, the latter becomes considerably smaller, probably as the result of the absorption of matters of nutritive value into other tissues. The final result is the formation of the compact "brown body," whose colour is principally derived from the pigment formerly present in the alimentary canal. Experiments made by introducing into the tissues of the Polyzoa certain artificial pigments which are known to be excreted by the kidneys when injected into the bodies of other animals, have given some reason for believing that the appearance of the brown pigment in the wall of the digestive organs is, in part, a normal process of excretion; although that process is not entirely carried out by the organs in question.
Little is known with regard to the duration of life of a single polypide; but some information bearing on this question may be obtained from a set of observations made on Flustra papyrea.[[508]] The table gives the number of days from the time at which the polypides were noticed to commence their degeneration:—