In Rhabdopleura each zooid forms its own delicate tube composed of a characteristic series of distinct rings.
In correspondence with the fundamental constitution of the zooid, each of the three segments has its own body-cavity separated from the others.
In C. nigrescens and in some other species a zooid may contain a pair of ovaries, a pair of testes, or an ovary and a testis, although the males, females and hermaj phrodites do not differ from one another in external characters.
- Zooid of tinued by its progeny, and thus gives Plumatella, with exrise to the mature colony.
On the degeneration of the polypide, its nutritive material is apparently absorbed for the benefit of the zooid, while the pig mented substances assume a spheroidal form, which either remains as an inert "brown body" in the body-cavity or is discharged to the exterior by the alimentary canal of the new polypide.
- Zooid of Paludicella articulata (= ehrenbergi).
The individual animal or zooid of Actinia equina has the form of a column fixed by one extremity, called the base, to a rock or other object, and bearing at the opposite extremity a crown of tentacles.
The elongation of the mouth and stomodaeum confer a bilateral symmetry on the body of the zooid, which is extended to other organs of the body.
3) has very constant anatomical characters, differing in some important respects from the Actinian zooid, which has been taken as a type.
The most simple form of budding is found in the genus Cornularia, in which the mother zooid gives off from its base one or more simple radiciform outgrowths.
Each outgrowth contains a single tube or solenium, and at a longer or shorter distance from the mother zooid a daughter zooid is formed as a bud.
- An expanded Alcyonarian zooid, showing the mouth surrounded by eight pinnate tentacles.
Further complications arise when the lower walls of the mother zooid become thickened and interpenetrated with solenia, from which buds are developed, so that lobose, tufted, or branched colonies are formed.The chief orders of the Synalcyonacea are founded upon the different architectural features of colonies produced by different modes of budding.
In the Tubiporidae the spicules of the proximal part of the body-wall are fused together to form a firm tube, the corallite, into which the distal part of the zooid can be retracted.
In this order the axis is formed as an ingrowth of the ectoderm of the base of the mother zooid of the colony, the cavity of the ingrowth being filled by a horny substance secreted by the ectoderm.
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.
B, Single zooid with the adjacent soft tissues as seen after removal of the skeleton by decalcification.
Z', the distal, and Z 2, the proximal or intracalicular portion of the zooid; ec, ectoderm; ct, coenenchymal tubes; sp, superficial network of solenia.
The structure of the zooid of Heliopora, however, is that of a typical Alcyonarian, and the septa have only a resemblance to, but no real homology with, the similarly named structures in madreporarian corals.
A pause in the development follows, during which no new mesenteries are formed, and then the six-rayed symmetry characteristic of a normal Actinian zooid is completed by the formation of the mesenteries V, V in the lateral chambers, and VI, VI in the sulcolateral chambers, their muscle-banners being so disposed that they form couples respectively with II, II and I, I.
As far as the anatomy of the zooid is concerned, the majority of the stony or madreporarian corals agree exactly with the soft-bodied Actinians, such as Actinia equina, both in the number and arrange 4 4 ¢ 2 4 FIG.
Each zooid has six tentacles; the stomodaeum is elongate, but the sulcus and sulculus are very feebly represented.
B, Single zooid and axis of the same magnified.
15) is a good example - exhibits the followings parts: - (I) The basal plate, between the zooid and the surface of attachment.
Though the corallum appears to live within the zooid, it is morphologically external to it, as is best shown by its developmental history.
- Transverse section through a zooid of Cladocora.
16 and 17 that every ec septum is covered by a fold of endoderm, mesogloea, and ectoderm, and is in fact pushed into the cavity of the zooid from without.
The zooid then is, as it were, moulded upon the corallum.
When fully extended, the upper part of the zooid projects for some distance out of the calicle, and its wall is reflected for some distance over the lip of the latter, forming a fold of soft tissue extending to a greater or less distance over the theca, and containing in most cases a cavity continuous over the lip of the calicle with the coelenteron.
A, Schematic longitudinal section through a zooid and bud of Stylophora digitata.
B, Similar section through a single zooid and bud of Astroides calicularis.
(C original; the rest after von Koch.) aporose corals, the only communication between the cavity of the edge-zone and the general cavity of the zooid is by way of the lip of the calicle; in the latter, or perforate corals, the theca is permeated by numerous branching and anastomosing canals lined by endoderm, which place the cavity of the edge-zone in communication with the general cavity of the zooid.
In the former case the young daughter zooid, with its corallum, arises wholly outside the cavity of the parent zooid, and the component parts of the young corallum, septa, theca, columella, &c., are formed anew in every individual produced.
In division a vertical constriction divides a zooid into two equal or unequal parts, and the several parts of the two corals thus produced are severally derived from the corresponding parts of the dividing corallum.
In colonial corals a bud is always formed from the edge-zone, and this bud develops into a new zooid with its corallum.
Buds arise from the edge-zone which already communicate with the cavity of the zooid by the canals.
As the buds develop the canal system becomes much extended, and calcareous tissue is deposited between the network of canals, the confluent edgezones of mother zooid and bud forming a coenosarc. As the process continues a number of calicles are formed, imbedded in a spongy tissue in which the canals ramify, and it is impossible to say where the theca of one corallite ends and that of another begins.
Massive meandrine corals are produced by continual repetition of a process of incomplete division, involving the mouth and to some extent the peristome: the calyx, however, does not divide, but elongates to form a characteristic meandrine channel containing several zooid mouths.
A Monograptus makes its first appearance as a minute dagger-like body (the sicula), which represents the flattened covering of the primary or embryonic zooid of the colony.
From this bud is developed the first zooid and first serial theca of the colony.
Each zooid buds from the one immediately preceding it in the series, and intercommunication is effected by all the budding orifices (including that in the wall of the sicula) remaining permanently open.
The theory of the gonangial nature of the vesicular bodies in the Graptoloidea is, however, disputed by some authorities, and it has been suggested that the zooid of the sicula itself is not the 20, Climacograptus Scharenbergi.