Helping San Diego, California and beyond since 1997.
Do you need
volunteer, community service, work, military or court hours?
|
|
Helping San Diego, California and beyond since 1997.
|
|
Click here and add this page to your favorites!
|
Encyclopedia Britannica - Main :: CLI-COM |
|
|
COELENTERA , a group or grade of the animal kingdom, the zoological importance of which has risen considerably since the time (1887) of the publication of the first article under that heading in the Ency. Brit. (9th edit.), even though their numbers have been reduced by the elevation
From the more complex colonial Protozoa the Coelentera are readily separated by their possession of two distinct sets of cells, with diverse functions, arranged in two definite layers,a condition found in no Protozoan. The old criterion by which they and other Metazoa were once distinguished from Protozoa, namely, the differentiation of large and small sexual cells from each other and from the remaining cells of the body
original
body
In the Coelentera the ectoderm and endoderm are set apart from one another at a very early period in the life-history; generally either by delamination or invagination, processes described in the article EMBRYOLOGY. Between these two cell-layers a mesogloea (G. C. Bourne, 1887) is always intercalated as a secretion by one or both of them; this is a gelatinoid, primitively structureless lamella, which in the first instance serves merely as a basal support for the cells. In many cases, as, for example, in the Medusae or jelly-fish, the mesogloea may be so thick as to constitute the chief
weight
function
The Coelentera may thus be briefly defined as Metazoa which exhibit two embryonic cell-layers only,the ectoderm and endoderm,their body-cavities being referable to a single cavity or coelenteron in the endoderm. Their position in the animal kingdom and their main subdivisions may be expressed in the following table: I. PROTOZOA. II. PARAZOA Or PORIFERA. Hydromedusae. VI. 9I In the above-given classification, the Scyphomedusae, formerly included with the Hydromedusae as Hydrozoa, are placed nearer the Anthozoa. The reasons for this may be stated briefly. The HYDROMEDUSAE are distinguished from the Scyphozoa chiefly by negative characters; they have no stomodaeum, that is, no ingrowth of ectoderm at the mouth to form an oesophagus; they have no mesenteries (radiating partitions) which incompletely subdivide the coelenteron; and they have no concentration of digestive cells into special
wall
The SCYLHOZOA have the following features in common:They typically exhibit an ectodermal stomodaeum; partitions or mesenteries project into their coelenteron from the body- wall
The SCYPHOMEDUSAE, like the Hydromedusae, typically present a metagenesis, the non-sexual scyphistomoid (corresponding to the hydroid) alternating with the sexual medusoid. In other cases the medusoid is hypogenetic, medusoid producing medusoid. The sexual cells of the medusoid lie in the endoderm on interradii, that is, on the second set of radii accentuated in the course of development. The medusoids have no true velum; in some cases a structure more or less resembling this organ, termed a velarium, is present, permeated by endodermal canals. The ANTHOZOA differ from the Scyphomedusae in having no medusoid form; they all more or less resemble a sea-anemone, and may be termed actinioid. They are (with rare exceptions, probably secondarily acquired) hypogenetic, the offspring resembling the parent, and both being sexual. The sexual cells are borne on the mesenteries in positions irrespective of obvious developmental radii. The CTENOPHORA are so aberrant in structure that it has been proposed to separate
As regards the other three groups, however, it is easy to conceive of them as derived from an ancestor, represented to-day to some extent by the planula-larva, which was Coelenterate in so far as it was composed of an ectoderm and endoderm, and had an internal digestive cavity (I. of the table). At the point of divergence between Scyphozoa and Hydromedusae (II. of the table of hypothetical descent), we may conceive of its descendant as tentaculate, capable of either floating ( swimming ) or fixation at will like Lucernaria to-day; and exhibiting incipient differentiation of myoepithelial cells (formerly termed neuro-muscular cells). At the parting of the ways which led, on the one hand, to modern Scyphomedusae, on the other to Anthozoa (III.), it is probable that the common ancestor was marked by incipient mesenteries and by the limitation of the sexual cells to endoderm. The lines of descentII. to Hydromedusae, and III. to Scyphomedusaerepresent periods during which the hypothetical ancestors II. and III., capable of either locomotion or fixation at will, were either differentiated into alternating generations of fixed sterile nutritive hydroids (scyphistomoids) and locomotor sexual medusoids, or abandoned the power of fixation in hypogenetic cases. During the periodII I Ceolentera Triploblastica = Diploblastica. (including Coelomata). Scyphozoa. Scyphomedusae. Anthozoa. Ctenophora. represented by the line of descentIII. to Anthozoathis group abandoned its power of adult locomotion by swimming . DuringI. these periods were also attained those less important structural characters which these three groups present to-day. (G.H.Fo.) End of Article: COELENTERA If you wish, you can link directly to this article.
<a href="http://jcsm.org/StudyCenter/Encyclopedia/CLI_COM/COELENTERA.html"> COELENTERA </a> |
|
|
(Previous) COEHOORN, MENNO, BARON VAN (1641-1704) |
(Next) COELLO, ALONSO SANCHEZ (1515-1590) |
|
Sponsored Advertisements