To establish the exact relationship it is necessary not only to breed but to rear the medusa, which cannot always be done in 1 In some cases hydroids have been reared in aquaria from ova of medusae, but these hydroids have not yet been found in the sea (Browne [Io a]).
In the Calyptoblastea the perisarc is always continued above the From Allman's Gymnoblastic Hydroids, by permission of the Council of the Ray Society.
Protohydra occurs in oysterbanks and Monobrachium also grows on the shells of bivalves, and both these hydroids probably fish in the currents produced by the lamellibranchs.
From Allman's Gymnoblastic Hydroids, by permission of the Council of the Ray Society.
The buds may all become detached after a time and give rise to separate and independent individuals, as in the common Hydra, in which only polyp-individuals are produced and sexual elements From Allman's Gymnoblastic Hydroids, by permission of are developed the Council of the Ray Society.
From Allman's Gymnoblastic mnoblastic Hydroids, by permission of the Council of the Ray Society.
Hence, in a colony of gymnoblastic hydroids, the oldest polyp of each system, that is to say, of the main stem or of a branch, is the topmost polyp; II ?a ` FIG.
Hence, in a colony of calyptoblastic hydroids, the oldest polyp of a system is the lowest; the youngest polyp is the top F =most one; and the axis of the system is a false axis composed of portions of each of the consecutive polyps.
The laws of branch formed by similar budding from 1; budding in hydroids a 2 -d 2 from 2, and so forth.
In some species of the genus Cunina (Narcomedusae) the youngest individuals (actinulae) are parasitic on other medusae (see below), but in later life the parasitic From Allman's Gymnoblastic Hydroids, by permission of habit is abandoned.
The histology described above for the polyp may be taken as the primitive type, from which that From Allman's G y mnoblastic Hydroids, by permission of the Council of the Ray Society.
In some hydroids the founder-polyp, developed from a planula after fixation, throws out numerous outgrowths from the base to form the hydrorhiza; these outgrowths may be radially arranged so as to form by contact or coalescence a flat plate.
The evidence against this view may be classed under two heads: first, comparative evidence; hydroids very different in their structural characters and widely separate in the systematic classification of these organisms may produce medusae very similar, at least so far as the essential features of medusan organization are concerned; on the other hydroids closely allied, perhaps almost indistinguishable, may produce gonophores in the one case, medusae in the other; for example, Hydractinia (gonophores) and Podocoryne (medusae), Tubularia (gonophores) and Ectopleura (medusae), Coryne (gonophores) and Syncoryne (medusae),-and so on.
Moreover, P. Hallez , has recently shown that hydroids hitherto regarded as distinct species are only forms of „ the same species grown under different conditions.
Until quite recently the hydroids (Gymnoblastea) and the medusae (Anthomedusae) have been classified separately, since the connexion between them was insufficiently known.
Other hydroids are Garveia, Bimeria, Eudendrium and Heterocordyle, with gonophores, and Dicoryne with peculiar sporosacs.
In this sub-order have received Such are the " snake-like zoids " and as such are generally inter 4"0 ' 'Y p P After Allman, Gymnoblastic Hydroids, by permission of the council of the Ray Society.
After Allman, Gymnoblastic Hydroids, by permission of the council of the Ray Society.
Two of the commonest British hydroids belong to this family, Obelia and Clytia.
Thus from the original planula three appendages are, as it were, budded off, while the planula itself mostly gives rise to coenosarc, just as in some hydroids the planula is converted chiefly into hydrorhiza.
Allman, " A Monograph of the Gymnoblastic or Tubularian Hydroids," Ray Society (1871-1872); 2.
Browne, " On British Hydroids and Medusae," Proc. Zool.
C. Mereschkowsky, " On a new Genus of Hydroids (Monobrachium) from the White Sea, with a short description of other new Hydroids," Ann.
Such differentiated water-conducting cells we call hydroids, the tissue they form hydrom.
Associated with the conducting parenchyma are frequently found hydroids identical in character with those of the central strand of the stem, and no doubt serving to conduct water to or from the leaf according as the latter is acting as a transpiring or a waterabsorbing organ.
The latter has a central strand consisting of files of large hydroids, separated from one another by very thin walls, each file being separated from its neighbor by stout, dark-brown walls.
At the periphery of the lobes are some comparatively thin-walled living cells mixed with a few thin-walled hydroids, the remains of the thin-walled hydrom mantle of the aerial stem.
In Dawsonia superba, a large New Zealand moss, the hydroids of the central cylinder of the aerial stem are mixed with thick-walled stereids forming a hydrom-stereom strand somewhat like that of the rhizome in other Polytrichaceae.
The whole of the middle lamella or originally formed cell-wall separating one from another disappears before the adult state is reached, so that the walls of the hydroids consist of a framework of lignified bars, with open communication between the cell cavities.
Grosvenor these nematocysts are derived from the hydroids on which the animals feed.
Orders: Ciliati (Rotifera), Denudati (Hydroids), Vaginati (Anthozoa and Polyzoa), Natantes (Crinoids).
The Hydrozoa comprise the hydroids, so abundant on all shores, most of which resemble vegetable organisms to the unassisted eye; the hydrocorallines, which, as their name implies, have a massive stony skeleton and resemble corals; the jelly-fishes so called; and the Siphonophora, of which the species best known by repute is the so-called "Portuguese man-of-war" (Physalia), dreaded by sailors on account of its terrible stinging powers.
A good example of this is seen in two common genera of British hydroids, Cordylophora and Tabularia.
Allman, "A Monograph of the Gymnoblastic or Tubularian Hydroids," Ray.
The shell-bearing forms belonging to this group which were known to Linnaeus were placed by him (in 1748) in the third order of his class Vermes under the name " Testacea," whilst the Echinoderms, Hydroids and Annelids, with the naked Mollusca, formed his second order termed " Zoophyta."
In 1830 he pointed out that among the numerous kinds of " polyps " at that time associated by naturalists with the Hydroids, there were many which had a peculiar and more elaborate type of organization, and for these he proposed the name Polyzoa.
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.
Allman, A Monograph of the Gymnoblastic or Tubularian Hydroids (Ray Society, 1871-1872).