Associated with these males are neuter zooids, which usually possess no functional repro ductive organs, but have in I -...
Hence it is necessary to distin guishbetween,first,the"zooids," FIG.
- Dasychone zooids are formed from a zone of budding 4.
Ultimately, a chain of sexual Malmgren.) zooids is thus formed.
A given stock only produces zooids of one sex.
Zooids possessing a high degree of individuality.
Zooids usually connected laterally with their neighbours.
Body-cavities of zooids not continuous with one another.
Body-cavities of zooids continuous with one another.
- Part of the creeping stolon, with zooids, of Pedicellina belgica.
A, c, Stalks of zooids of different ages; b, bud.
Ctenostomata the colony is similarly constituted, a branched stolon giving off the zooids, which are not connected with one another.
In the majority of Ectoprocta there is no stolon, the zooids growing out of one another and being usually apposed so as to form continuous sheets or branches.
In the encrusting type, which is found in a large proportion of the genera, the zooids are usually in a single layer, with their orifices facing away from the substratum; but in certain species the colony becomes multilaminar by the continued superposition of new zooids over the free surfaces of the older ones, whose orifices they naturally occlude.
The zoarium may rise up into erect growths composed of a single layer of zooids, the orifices of which are all on one surface, or of two layers of zooids placed back to back, with the orifices on both sides of the fronds or plates.
The zooids of which the colonies of Ectoprocta are composed consist of two parts: the body-wall and the visceral mass (figs.
In Actinia, as in all Anthozoan zooids, the coelenteron is not a simple cavity, as in a Hydroid, but is divided by a number of radial folds or curtains of soft tissue into a corresponding number of radial chambers.
The only exceptions to this structure are found in the arrested or modified zooids, which occur in many of the colonial Alcyonaria.
Such modified zooids are called siphonozooids, their function being to drive currents of fluid through the canal-systems of the colonies to which they belong.
Zooids arise at intervals.
In the order Stolonifera the zooids spring at intervals from branching or lamellar stolons, and are usually free from one another, except at their bases, but in some cases horizontal solenia arising at various heights from the body-wall may place the more distal portions of the zooids in communication with one another.
Of bunches of elongate cylindrical zooids, whose proximal portions are united by solenia and compacted, by fusion of their own walls and those of the solenia, into a fleshy mass called the coenenchyma.
Thus the coenenchyma forms a stem, sometimes branched, from the surface of which the free portions of the zooids project.
In the order Pseudaxonia the colonies are upright and branched, consisting of a number of short zooids whose proximal ends are imbedded in a coenenchyma containing numerous ramifying solenia and spicules.
The latter contains the proximal moieties of the zooids and numerous but separate spicules.
It owes its commercial value to the beauty of its hard red calcareous axis which in life is covered by a cortex in which the proximal moieties of the zooids are imbedded.
In the order Stelechotokea the colony consists of a stem formed by a greatlyelongated mother zooid, and the daughter zooids are borne as lateral buds on the stem.
They resemble and are closely allied to certain families of the Cornulariidae, differing from them only in mode of budding and in the disposition of the daughter zooids round a central, much-elongated mother zooid.
The prorachidial and metarachidial aspects of the rachis are sterile, but the sides or pararachides bear numerous daughter zooids of two kinds - (I) fully-formed autozooids, (2) small stunted siphonozooids.
The pinnae are formed by the elongated autozooids, whose proximal portions are fused together to form a leaf-like expansion, from the upper edge of which the distal extremities of the zooids project.
Larger star-shaped cavities, called calices, in which the zooids are lodged, and very numerous smaller round or polygonal apertures, which in life contain as many short unbranched A FIG.
14), forms arborescent colonies consisting of numerous zooids arranged in a single series along one surface of a branched horny axis.
After division the corallites continue to grow upwards, and their zooids may remain united by a bridge of soft tissue or coenosarc. But in some cases, as they grow farther apart, this continuity is broken, each corallite has its own edge-zone, and internal continuity is also broken by the formation of dissepiments within each calicle, all organic connexion between the two zooids being eventually lost.
The lastnamed method has proved little more than that there is a remarkable similarity between the zooids of all recent corals, the differences which have been brought to light being for the most part secondary and valueless for classificatory purposes.
On the other hand, the study of the anatomy and development of the zooids has thrown much light upon the manner in which the corallum is formed, and it is now possible to infer the structure of the soft parts from a microscopical examination of the septa, theca, &c., with the result that unexpected relationships have been shown to exist between corals previously supposed to stand far apart.
Duerden has shown that in Porites the ordinary zooids possess only six couples of mesenteries arranged on the Actinian plan.
But some zooids grow to a larger size and develop a number of additional mesenteries, which arise either in the sulcar or the sulcular entocoele, much in the same manner as in Cerianthus.
Each Graptolite represents the common horny or chitinous investment or supporting structure of a colony of zooids, each tooth-like projection marking the position of the sheath or theca of an individual zooid.
In Dictyonema the branches show thecae of two kinds: (I) the ordinary tubular thecae answering to those of the Graptoloidea and occupied by the nourishing zooids; and (2) the so-called bithecae, birdnest-like cups (regarded by their discoverers as gonothecae) opening alternately right and left of the ordinary thecae.