Hydromedusae Sentence Examples
The Hydromedusae form a widespread, dominant and highly differentiated group of animals, typically marine, and found in all seas and in all zones of marine life.
In fresh-water Hydromedusae the life-cycle is usually secondarily simplified, but in marine forms the life-cycle may be extremely complicated, and a given species often passes in the course of its history through widely different forms adapted to different habitats and modes of life.
At present, therefore, classifications of the Hydromedusae have a more or less tentative character, and are liable to revision with increased knowledge of the life-histories of these organisms. Many groups bear at present two names, the one representing the group as defined by polyp-characters, the other as defined by medusa-characters.
As already stated, there occur in the Hydromedusae two distinct types of person, the polyp and the medusa; and either of them is capable of non-sexual reproduction by budding, a process which may lead to the formation of colonies, composed of more or fewer individuals combined and connected together.
The ganglion-cells of Hydromedusae are generally very small.
As regards habit of life the vast majority of Hydromedusae arc 6 FIG.
Th internal anatomy of the Hydromedusae shows numerous variations.
The nervous system of the medusa consists of sub-epithelial ganglion-cells, which form, in the first place, a diffuse plexus of nervous tissue, as in the polyp, but developed chiefly on the subumbral surface; and which are concentrated, in the second place, to form a definite central nervous system, never found in the polyp. In Hydromedusae the central nervous system forms two concentric nerverings at the margin of the umbrella, near the base of the velum.
In Hydromedusae the sense-organs are always exposed at the umbrellar margin (hence Gymnophthalmata), while in Scyphomedusae they are covered over by flaps of the umbrellar margin (hence Steganophthalmata).
Beyond this simple condition the visual organs of the Hydromedusae do not advance, and are far from reaching the wonderful development of the eyes of Scyphomedusae (Charybdaea).Advertisement
The reproductive cells may be regarded as belonging primarily to neither ectoderm nor endoderm, though lodged in the ectoderm in all Hydromedusae.
In the Hydromedusae they usually, if not invariably, ripen in the ectoderm, but in the neighbourhood of the main sources of nutriment, that is to say, not far from the stomach.
Nearly every possible method of reproduction occurs amongst the Hydromedusae.
The only one of these possible modes of reproduction not known to occur in Hydromedusae is parthenogenesis.
The ovum of Hydromedusae is usually one of a large number of odgonia, and grows at the expense of its sister-cells.Advertisement
To complete our survey of life-cycles in the Hydromedusae it is necessary to add a few words about the position of Hydra and its allies.
There is no difficulty whatever in regarding Hydra as bearing the same relation to the actinula-stage of other Hydromedusae that a Rotifer bears to a trochophore-larva or a fish to a tadpole.
Many views have been put forward as to the morphological relationship between the two types of person in the Hydromedusae.
The question still remains open, however, which of the two types of person may be regarded as the most primitive, the most ancient in the race-history of the Hydromedusae.
Brooks, on the other hand, as stated above, regards the medusa as the older type and looks upon both polyp and medusa, in the Hydromedusae, as derived from a free-swimming or floating actinula, the polyp being thus merely a fixed nutritive stage, possessing secondarily acquired powers of multiplication by budding.Advertisement
The Narcomedusae exhibit peculiarities of form and structure which distinguish them at once from all other Hydromedusae.
Solmaris, Pegantha, Polyxenia, &c. To this family should be referred, probably, the genus Hydroctena, described by C. Dawydov [11a] and regarded by him as intermediate between Hydromedusae and Ctenophora.
Putting aside the last-named, for a detailed account of which see Hydromedusae, we can best deal with the peculiarities of the polyp and medusa from a developmental point of view.
The class is divisible into two main divisions or sub-classes, Hydromedusae and Scyphornedusae, of which definitions and detailed systematic accounts will be found under these headings.
From the stomach, canals arise termed the radial canals (r.c.); typically four in number, they run in a radial direction to the edge 2 For other variations of the medusa, often of importance for systematic classification, see Hydromedusae and Scyphomedusae.Advertisement
In this way complicated cycles of alternating generations arise, which are described fully in Hydromedusae and Scyphomedusae.
In the above-given classification, the Scyphomedusae, formerly included with the Hydromedusae as Hydrozoa, are placed nearer the Anthozoa.
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 organs.
The Scyphomedusae, like the Hydromedusae, typically present a metagenesis, the non-sexual scyphistomoid (corresponding to the hydroid) alternating with the sexual medusoid.
The umbrella has a lobed, indented margin, a character only seen amongst Hydromedusae in the order Narcomedusae, and it is without the characteristic velum of the Hydromedusae; hence the Scyphomedusae are sometimes termed Hydrozoa Acraspeda.
A true velum, such as is found in Hydromedusae, never contains endoderm.
The nervous system consists as in Hydromedusae of a diffuse plexus beneath the ectoderm, concentrated in certain places to form a central nervous system.
A similar origin for metagenesis has been discussed under the Hydromedusae (q.v.).
There remains only the third feature, the endodermal gonads, as an argument for uniting the Scyphomedusae with the Anthozoa, against which must be set all the peculiarities of medusan organization in which the Scyphomedusae resemble the Hydromedusae.
The fact that the Scyphomedusae have a number of well-marked peculiarities of form and structure is not incompatible with placing them in the Hydrozoa as a distinct sub-class, contrasting sharply in many ways with the Hydromedusae.
For further details of colonyformation the reader is referred to the articles Anthozoa and Hydromedusae.
At one time they were referred by some to the Polyzoa (Bryozoa), and later, by almost general consent, to the Hydroida (Calyptoblastea) among the Hydrozoa (Hydromedusae).
The muscular tissue of the Hydromedusae is entirely ectodermal.
As has been described above, the endoderm may also contribute to the sense-organs, but such contributions are always of an accessory nature, for instance, concrement-cells in the otocysts, pigment in the ocelli, and never of sensory nature, sense-cells being Hydromedusae are of separate sexes, the only known exception being Amphogona apsteini, one of the Trachomedusae (Browne ).
It will be seen elsewhere, however, that whatever view may be held as to the origin of metagenesis in Hydromedusae, in the case of Scyphomedusae (q.v.) no other view is possible than that the alternation of generations is the direct result of larval proliferation.