Zooxanthellae

“ Among the Madreporaria we find great branching colonies of thousands of zooids supported by the copious skeleton of carbonate of lime that they have secreted. Among the Antipathidea, again, we find a dendritic skeleton of a dark horny substance, formed by a colony of small zooids that cover it like a thin bark. The majority of the Zoantharia are, like other zoophytes, permanently fixed to the floor of the ocean. Where the embryo settles, there must the adult or colony of adults remain until death. ”

“ In some, a symbiosis occurs with Zooxanthella, Brandt, a Flagellate of the group Chrysomadineae, which in the resting state inhabits the extracapsular cytoplasm growing and dividing freely therein, and only (under study) becoming free and flagellate on the death of the host (fig. III. 4, 6-13) . ”

“ The sexual reproduction of a great many species of Zoantharia has now been observed. The eggs are, as a general rule, ripened in batches, and fertilisation is effected before their discharge from the body. In some cases the sexual condition is seasonal. In temperate climates the generative organs ripen in the spring and {373}summer months, and remain small and relatively inconspicuous in the colder weather; but British Sea-anemones, when kept in an aquarium and regularly fed, will breed nearly all the year round. ”

“ Major Walton found it at Bulandshahr in the United Provinces, and it is not uncommon in the neighbourhood of Calcutta.
Biology.—The zoaria of S. indica are usually fixed to the roots of duckweed or to the stems of other plants. They are often found together with those of P. punctata. A slight infusion of brackish water into the ponds in which it lives does not seem to be inimical to this species, but I have found it in ponds in which nothing of the kind was possible. It flourishes during the "rains" and, to judge from specimens kept in an aquarium, is very short-lived. ”

“ The form of the Xanthellæ is usually spherical and elliptical, often also sphæroidal or discoidal. Their diameter is usually between 0.008 and 0.012 mm., rarely more or less. The differences exhibited by Xanthellæ which live in different groups of Radiolaria demand further investigation, which will perhaps lead to the establishment of several species of the genus Zooxanthella. At present Zooxanthella extracapsularis, in the calymma of Spumellaria and Nassellaria, may be clearly distinguished from Zooxanthella intracapsularis, in the central capsule of the Acantharia. ”

“ All the orders, with the exception of the Antipathidea and Zoanthidea, contain genera of solitary zooids, and the orders Edwardsiidea and Cerianthidea contain no genera that form colonies. In the Madreporaria, Zoanthidea, and Antipathidea, on the other hand, colonies are formed composed of a very large number of individuals which frequently attain to a very great size. The term "Sea-anemone" is commonly used in writing about the solitary Zoantharia which do not form any skeletal structures, and the term "Coral" is applied to all those Zoantharia which do form a skeleton. ”

“ Marine animals commonly produce far more eggs than insects. The dangers of the shallow seas are so great that a small proportion only of the young animals come to maturity. Hence the enormous fertility of common marine animals, except such as are able to nourish or defend their young. Vast numbers of Zoœa are swept into mid-ocean or into tidal rivers, or are devoured. It is only a chance remnant that survives. [5] Prof. Möbius says that out of a million oyster embryos only one individual grows up, a mortality due to untoward currents and surroundings, as well as to hungry mouths. ”

“ In Cristatella we have the most highly modified of all the Phylactolaemata. The individuality of the zooecium is here subordinated to that of the colony as a whole. The branched arrangement of the zooecia is greatly obscured. The body-cavities have become completely confluent, although rudiments of the septa still exist. The ectocyst has been lost, with the exception of the basal layer of the colony. The tentacles are more numerous (80-90) ; and in accordance with the increase in the elaboration of the genus, its statoblasts belong to the most complicated type known. ”

“ Such are the beautiful family of the Annelidae: the Serpula, in his dirty house; and the Terebella, most ancient of masons, who lays the walls of his home in water-proof cement.
The great class of Zoöphytes can be introduced, but many varieties of them will be found already within the aquarium, in the company of their more bulky neighbors. These peculiar creatures, or things, form the boundary where the last gleam of animal life is so feeble and flickering as to render it doubtful whether they belong to the animal or vegetable kingdom. Agassiz calls them Protozoa,—Primary Existences. ”

“ There are six complete mesenteries in each zooid, but as they bear no retractor muscles it is not certain that they represent the first six protocnemes of other Zoantharia. In a great many species the zooids are oval in shape, the longer diameter being parallel with the axis of the branch. ”

“ These three forms, Foraminifera, Xanthidia, and spicules of sponge represent the chief varieties of organic remains found in the flint, each of them abounded in the waters of the chalk ocean, and the problem before us therefore narrows itself to the consideration how they were first collected and compacted into the plastic masses of siliceous matter which have hardened into the pebbles we daily tread upon. A little study of the life history of one of these organisms, the Xanthidium vestitum, may help us towards an answer. ”

“ The avicularia of some of the zooecia have been omitted in B.
General Characters.—The Polyzoa are colonies, leaf-like or tree-like in form, and often strongly resembling seaweeds, or forming encrustations on the surface of stones and water-plants, or taking on other shapes. The units of the colony are complete individuals (Fig. 234) . The zooecium or body-wall encloses a body-cavity, in which lies a digestive canal, with which are closely connected the central nervous system and the retractile, ciliated tentacles. The structures other than the zooecium constitute the "polypide." ”

“ The zoarium never covers a large area and as a rule grows sparingly and mainly in two directions.
(2) The zoœcia are more irregular in shape, not so distinctly elbowed, smaller; they have a much more prominently keeled ridge. The great majority of them are constricted at the base and taper towards the orifice. In young zoaria they are almost colourless but in older ones there is a band of not very dense pigment round the base of the vertical limb.
(3) The free statoblasts are comparatively large and usually show a tendency to taper at the extremities, often being almost rhomboidal in form. ”

“ The genus is widely distributed and extends from shallow water to depths of 1500 fathoms. Caryophyllia sometimes occurs in clusters which have the appearance of an incipient colony. This may be due to the embryos fixing themselves upon the epitheca of existing individuals and developing there. It is doubtful whether the species ever reproduce asexually either by gemmation or by fission. When the zooid is fully expanded it projects some distance above the corallum and shows a very transparent body-wall with a crown of some fifty tentacles. ”

“ Biology.—The zoaria of the species of Plumatella are found firmly attached to stones, bricks, logs of wood, sticks, floating seeds, the stems and roots of water-plants, and occasionally to the shells of molluscs such as Vivipara and Unio. Some species shun the light, but all are apparently confined to shallow water. ”

“ In certain genera, however, and particularly in some Ctenostomes (Fig. 238) , and in most of the Entoprocta, the {480}individuals grow out at intervals from a cylindrical stem or "stolon" (st) , which is not composed of zooecia.
The Cyclostomata may assume an erect or encrusting habit. Their zooecia are always more or less cylindrical; the upper ends being often completely free, although in many cases the whole zooecium is closely adnate to its neighbours. In the breeding season the forms which belong to this group are provided with curious "ovicells," which contain the embryos. ”

“ Zoochlorella (Brandt) , is the name given to undetermined Chlamydomonads found multiplying in the resting state within and in symbiotic relation to other Protozoa, to the freshwater sponge, Ephydatia, Hydra viridis, and to the Turbellarian, Convoluta viridis (in which last species the active form has been recognized as a Carteria) .
Family 2.—Volvocidae. Cell-wall gelatinous; always associated in colonies; cells, as in Family 1. The number of individuals united to form a colony varies very much, as does the shape of the colony. ”

“ Other forms go further and secrete a gummy substance that glues the upright zoœcia [Pg 214] together and forces them to elongate themselves without branching. In these conditions the zoœcia become polygonal in cross-section. It is probable that such forms (e. g., Plumatella fungosa (Pallas) ) should rank as distinct species, for the gummy secretion is present in great profusion even in young zoaria in which the zoœcia have not yet assumed a vertical position. No such form, however, has as yet been found in India, and in any case it is impossible to regard Alcyonella as a distinct genus. ”

“ The zoœcia are never distinct from one another, but in dendritic forms such as Plumatella the zoarium is divided at irregular intervals by chitinous partitions. The lophophore in most genera is horseshoe-shaped instead of circular, the part opposite the anus being deeply indented. There are no parietal muscles. The orifice of the zoœcium is always circular, and there is no trace of any structure corresponding to the collar of the ctenostomes. The tentacles are always webbed at the base. ”

“ There is hardly a more surprising spectacle in the whole animal kingdom than a living fragment of the genus Bugula. The colony grows in the shape of a small tree, whose height may amount to several inches; and is characterised, in many species, by a spiral arrangement of the branches, which makes the genus easy to recognise at first sight (Fig. 233, A) . The stem and branches are composed of a single layer of zooecia, arranged two or more abreast. Each zooecium bears, on its outer side, a most singular body termed an avicularium, from its resemblance to a bird's head. ”

“ Of the two Indian genera of this subfamily, one (Plumatella) is almost universally distributed, while the other (Stolella) has only been found in the valley of the Ganges. The third genus of the subfamily (Stephanella) is only known from Japan.
It should be noted that zoaria of different species and genera of this subfamily are often found in close proximity to one another and to zoaria of Fredericella, and that the branches of the different species are sometimes entangled together in such a way that they appear, unless carefully separated, to belong to the same zoarium. ”

“ In nearly all cases the fertilisation is effected, and segmentation of the ovum occurs within the body of the parent, the young Zoantharian beginning its independent life as an oval or pear-shaped ciliated larva.
There are a great many cases among the Actiniaria in which the embryos are retained within the coelenteron, or in special brood pouches of the parent (p. 379) , until a stage is reached with twelve or more tentacles.
The oval or pear-shaped larva swims about for a few days or hours, and then settles down on its aboral end. In swimming, the aboral end is always turned forwards. ”

“ Most of the species occur in deep water, some in very deep water (A. simplex, 1600 to 1700 fathoms) . In this and the following genera the coenenchym is {354}thin and the zooids imperfectly or not retractile. Ceratoisis, Wright; Atlantic Ocean, extending from shallow to deep water. The branches arise from the nodes. Chelidonisis, Studer; deep water off the Azores. Isidella, Gray; Mediterranean Sea. Bathygorgia, Wright; off Yokohama, 2300 fathoms. This genus is unbranched, with very long internodes and short nodes. The zooids are arranged on one side only of the stem. ”

“ The zoophytes which constitute the class Zoantharia are quite great personages. Some of them are eighteen or twenty inches long; at the same time, others scarcely exceed the eighth part of an inch in length. They live in all seas, and seem to have existed through many ages of the earth's history; they appear at an early geological period, and they have performed an important part in its formation; we shall see that, with great numbers of them, parts cut off from their bodies continue to live and become new individuals. ”

“ In the vast majority of the polyzoa, marine as well as freshwater, movement is practically confined to the polypide, the external walls of the zoœcium being rigid, the zoœcia being closely linked together and the whole zoarium permanently fixed to some extraneous object. In a few freshwater species belonging to the genera Cristatella, Lophopus, Lophopodella and Pectinatella, the whole zoarium has the power of progression. ”

“ V, Myrianida, Autolytus, etc. From this zone of budding a very large number of segments have been formed, which have, further, become grouped so as to form three individuals, C, D, E; B is the hindmost zooid, which is either formed from the hinder segments of the parent zooid or is produced by budding, like C, D, E.
One original "stock," or asexual zooid, thus produces several sexual zooids, but these are only of one sex for a given stock. The males differ in several important characters from the females ”