Theca Sentence Examples
A, A male gonophore still enclosed in its ecto theca.
A-D are stages common to both; from D arises the hydrotheca (E) or the gonotheca (F); th, theca; st, stomach; 1, tentacles; m, mouth; mb, medusa-buds.
In B the spadix of the upper bud has protruded itself through the top of the gonoat the base of the theca and the acrocyst (ac) is secreted round it.
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.
In some corals the septa are solid imperforate plates of calcite, and their peripheral ends are either firmly welded together, or are united by interstitial pieces so as to form imperforate theca.Advertisement
In others the peripheral ends of the septa are united only by bars or trabeculae, so that the theca is perforate, and in many such perforate corals the septa themselves are pierced by numerous perforations.
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.
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.
Corals have been divided into A porosa and Perforata, according as the theca and septa are compact and solid, or are perforated by pores containing canals lined by endoderm.
Various attempts have been made to classify corals according to the arrangement of the septa, the characters of the theca, the microscopic structure of the corallurn, and the anatomy of the soft parts.Advertisement
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.
Theca porous; septa compact and reduced in number.
From this bud is developed the first zooid and first serial theca of the colony.
This theca grows in the direction of the apex of the sicula, to which it adheres by its dorsal wall.
From this first theca originates a second, opening in the same direction, and from the second a third, and so on, in a continuous linear series until the polypary is complete.Advertisement
The sicula itself ceases to grow soon after the earliest theca have been.
The thecae in the earliest family - Dichograptidae - are so similar in form to the sicula itself that the polypary has been compared to a colony of siculae; there is the greatest variation in shape in those of the latest family - Monograptidae--in some species of which the terminal portion of each theca becomes isolated (Rastrites) and in some coiled into a rounded lobe.
That part of the theca below the origins of the free arms is called the "dorsal cup"; the ventral part above the origins of the arms, serving as cover to the cup, is known as the "tegmen."
Although, in the extreme correlation of the radial food-grooves, nerves, watervessels, and so forth, with a radiate symmetry of the theca, such a type differs from the Cystidea, while in the possession of jointed processes from the radial plates, bearing the grooves and the various body-systems outwards from the theca, it differs from all other Echinoderms, nevertheless ancient forms are known which, if they are not themselves the actual links, suggest how the crinoid type may have been evolved from some of the more regular cystids.
At last a quinqueradiate symmetry influenced the plates of the theca, partly through the development of a plate at the end of each groove (terminal), partly through plates at the aboral pole of the theca (basals and infrabasals) arising in response to mechanical pressure, but soon intimately connected with the cords of an aboral nervous system.Advertisement
Before the latter plates arose, the stem had developed by the elongation and constriction of the fixed end of the theca, the gradual regularization of the plates involved, and their coalescence into rings.
The crinoid type was differentiated by the extension of the food-grooves and associated organs along radial outgrowths from the theca itself.
These constituted the arms (brachia), and five definite radial plates of the theca were specialized for their support.
In this development of brachial extensions of the theca the genital organs were involved, and their ripe products formed at the ends of the brachia or in the branches therefrom.
The remains of the original genital gland within the theca became the "axial organ" surrounded by the "axial sinus" derived from the anterior coelom, and this again by structures derived from the right posterior coelom, which, as explained above, had been depressed to the aboral pole.Advertisement
The Protocrinidae lead up to Proteroblastus, in which the theca is ovoid, sometimes prolonged into a stem, the plates differentiated into (a) smooth, irregular, depressed interambulacrals, (b) transversely elongate brachioliferous adambulacrals, to which the diplopores, which lie at right angles to the main food-groove, are confined.
The fibrous theca externa merges with the surrounding stroma.
The nerve roots appear thickened and clumped together and lie around the periphery of the theca interna in the lower lumbar canal.