In many genera and larger groups the food commonly passes down the pharynx into the stomach without interruption, although it is probable that in all species the œsophagus can be closed off from the stomach by a valve at its base. In some forms, however, a "gizzard" is interposed between the œsophagus and the stomach. This gizzard has not the same function in all cases, for whereas in some forms (e. g., in Bowerbankia) it is lined with horny projections and is a powerful crushing organ, in others (e. g., in Hislopia or Victorella) it acts as an antechamber in which food can be preserved without being crushed until it is required for digestion, or rough indigestible particles can be retained which would injure the delicate walls of the stomach.
Digestion takes place mainly in the stomach, the walls of which are of a glandular nature. The excreta are formed into oval masses in the rectum and are extruded from the anus in this condition.
Although the gross non-nutritious parts of the food are passed per anum, the waste products of the vital processes are not eliminated so easily, and a remarkable process known as the formation of brown bodies frequently takes place. This process cannot be described more clearly and succinctly than by quoting Dr. Harmer's description of it from pp. 471 and 472 of vol. ii. of the Cambridge Natural History, a volume to which I have been much indebted in the preparation of this introduction. The description is based very largely on Dr. Harmer's own observations[[AW]].
"The tentacles, alimentary canal, and nervous system break down, and the tentacles cease to be capable of being protruded. The degenerating organs become compacted into a rounded mass, known from its colour as the 'brown body.' This structure may readily be seen in a large proportion of the zoœcia of transparent species. In active parts of the colony of the body-wall next develops an internal bud-like structure, which rapidly acquires the form of a new polypide. This takes the place originally occupied by the old polypide, while the latter may either remain in the zoœcium 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. The apex of the blind pouch or 'cæcum' of the young stomach is guided by this strand to the 'brown body,' which it partially surrounds. 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."
Brown bodies are rarely if ever found in the phylactolæmata, in which the life of the colony is always short; but they are not uncommon in Hislopia and Victorella, although in the case of the former they may easily escape notice on account of the fact that they are much paler in colour than is usually the case. When they are found in a ctenostome the collar-like membrane characteristic of the suborder is extruded from the orifice (which then disappears) and remains as a conspicuous external addition to the zoœcium, the ectocyst of which, at any rate in Bowerbankia and Victorella, sometimes becomes thickened and dark in colour.
It is noteworthy that the colouring matter of the brown bodies is practically the only colouring matter found in the polypides of most polyzoa. Young polypides are practically colourless in almost all cases.
Reproduction: Budding.
Polyzoa reproduce their species in three ways—(i) by means of eggs, (ii) by budding, and (iii) by means of bodies developed asexually and capable of lying dormant in unfavourable conditions without losing their vitality.
Most, if not all species are hermaphrodite, eggs and spermatozoa being produced either simultaneously or in succession by each individual, or by certain individuals in each zoarium. The reproductive organs are borne on the inner surface of the endocyst, as a rule in a definite position, and often in connection with the funiculus or funiculi. It is doubtful to what extent eggs are habitually fertilized by spermatozoa of the individual that has borne them, but in some cases this is practically impossible and spermatozoa from other individuals must be introduced into the zoœcium.
Budding as a rule does not result in the formation of independent organisms, but is rather comparable to the proliferation that has become the normal method of growth in sponges, except of course that individuality is much more marked in the component parts of a polyzoon colony than it is in a sponge. In the genera described in this volume budding takes place by the outgrowth of a part of the body-wall and the formation therein of a new polypide, but the order in which the buds appear and their arrangement in reference to the parent zoœcium is different in the different groups. In the freshwater ctenostomes three buds are typically produced from each zoœcium, one at the anterior end and one at either side, the two latter being exactly opposite one another. The parent zoœcium in this formation arises from another zoœcium situated immediately behind it, so that each zoœcium, except at the extremities of the zoarium, is connected with four other zoœcia, the five together forming a cross. The two lateral buds are, however, frequently suppressed, or only one of them is developed, and a linear series of zoœcia with occasional lateral branches is formed instead of a series of crosses. In the phylactolæmata, on the other hand, the linear method of budding is the typical one, but granddaughter-buds are produced long before the daughter-buds are mature, so that the zoœcia are frequently pressed together, and lateral buds are produced irregularly. In Victorella additional adventitious buds are produced freely near the tip of the zoœcium.