t "1j ' laterally from it; the result is an arborescent, tree-like colony (figs.

Certain discrepancies must also be noted.Trophosome, polyps with two whorls of tentacles, the lower filiform, the upper capitate; gonosome, free medusae, with tentacles solid and branched. The possession of an entocodon proves the medusa-nature of the bud,. The body-form of the Scyphomedusae varies from that of a conical nervous system does not form continuous rings, but occurs as four or

already stated, and - Hydrothecae sessile, biserial on the main stem, uniserial on the lateral branches or pinnules, which give the colony its characteristic feathery form; with nematophores.

Hydromedusae are smaller and more delicate than scyphomedusae or cubomedusae; they may be completely absent from the life cycle of some hydrozoan species.

?` distinguished; the first /??

There is no free-swimming planula larva, but the stage corresponding to it is passed over in an enveloping cyst, which is secreted round the embryo by its own ectodermal layer, shortly after the germ-layer formation is complete, In all the abovementioned genera, with the exception of The two kinds of persons present in the typical Hydroidea make the classification of the group extremely difficult, for reasons explained above. ), the other being the Hydromedusae (q.v.). Never more than one pneumatophore is found in a cormus, and when present it is always situated at the highest point above the swimming bells, if these are present also. By coalescence of the endoderm-layers, the coelenteron may be reduced to vessels, usually eight in number, opening into a ring-sinus surrounding the pore. Groups founded on polypcharacters are printed in ordinary type, those founded on medusacharacters in italics. What are commonly called jellyfish are medusae belonging to three Classes of the Cnidaria — the Hydrozoa, the Scyphozoa, and the Cubozoa. ScienceDirect ® is a registered trademark of Elsevier B.V.URL: https://www.sciencedirect.com/science/article/pii/B9780857091192500139URL: https://www.sciencedirect.com/science/article/pii/B9780123744739001983URL: https://www.sciencedirect.com/science/article/pii/S1937644810800024URL: https://www.sciencedirect.com/science/article/pii/B9780123850249000046URL: https://www.sciencedirect.com/science/article/pii/B9780123869197000186URL: https://www.sciencedirect.com/science/article/pii/B9780124080966000018URL: https://www.sciencedirect.com/science/article/pii/B978012394282100003XURL: https://www.sciencedirect.com/science/article/pii/S0065288103440029URL: https://www.sciencedirect.com/science/article/pii/B9780123877383000524URL: https://www.sciencedirect.com/science/article/pii/B9780124555211500103Nilsson et al., 2005; Piatigorsky et al., 1989; Weber, 1981Thorp and Covich's Freshwater Invertebrates (Fourth Edition)Biology and Ecology of Irukandji Jellyfish (Cnidaria: Cubozoa)Barnes, 1964, 1966; Cleland and Southcott, 1965; Kinsey, 1988Jellyfish Life Histories: Role of Polyps in Forming and Maintaining Scyphomedusa PopulationsCIESM, 2001; Mills, 2001; Parsons and Lalli, 2002; Richardson Schneider and Behrends, 1994; Lucas, 2001; Colin and Kremer, 2002Brewer and Feingold, 1991; Watanabe and Ishii, 2001; Prieto Matsakis and Conover, 1991; Nicholas and Frid, 1999; Purcell A Synthesis of Growth Rates in Marine Epipelagic Invertebrate ZooplanktonBlinks et al., 1982; Charbonneau et al., 1985; Cormier et al., 1989; Shimomura, 2006ScienceDirect ® is a registered trademark of Elsevier B.V. 49. The septal ostia become widened and the gastral cavity The light may be emitted as one or many flashes per stimulus. the other We can distinguish (I) digestive endoderm, in the stomach, often with special glandular elements; (2) circu-, latory endoderm, in the radial and ring canals; (3) supporting endoderm in the axes of the tentacles and in the endodermlamella; the latter is primitively a double layer of cells, produced by concrescence OC-- = w.?" 15),the polyps produce buds right and left alternately, so that the hydranths are arranged in a zigzag fashion, forming a " scorpioid cyme," as in In a colony formed by sympodial budding, a polyp always produces first a bud, which contributes to the system to which it belongs, As in other cases where animal colonies are formed by organic union of separate individuals, there is ever a tendency for the polyp-colony as a whole to act as a single individual, and for the members to become subordinated to the needs of the colony and to undergo specialization for particular functions, with the result that they simulate organs and their individuality becomes masked to a greater or less degree. After W. Schewiakoff, In the endoderm large concretions are formed (con.). of a peculiar type termed Ephyrae (fig.

- Portion of colony of the youngest polyp of the system is the one nearest to the topmost polyp; and the axis of the system is a true axis.In the sympodial method of budding, on the other hand, the founder-polyp is of limited growth, and forms a bud from its side, which is also of limited growth, and forms a bud in its turn, and so on (figs. The process of budding as above described may be varied or complicated in various ways; thus a secondary, amnion-like, ectodermal covering or ectotheca (fig. Many of the species produce medusae asexually from the hydroid stage, which lives attached to hard surfaces, including aquaculture structures. Weismann.) sense-organs The simplest type of ocellus is exemplified by the The sub-epithelial tissues, It has been shown above that polyps are budded only from polyps and that the medusae may be budded either from polyps or from medusae.

By a simple modification, the open pit becomes a solid ectodermal ingrowth, just as in Teleostean fishes the hollow medullary tube, or the auditory pit of other vertebrate embryos, is formed at first as a solid cord of cells, which acquires a cavity secondarily.

are always The stomach has sixteen marginal pouches and The pneumato phore arises from the ectoderm as a pit or invagination, part of which forms a gas-secreting gland, while the rest gives rise to an air-sack lined by a chitinous cuticle.

here we focus on scyphomedusae, cubomedusae, and hydromedusae.

68.

A-D are stages common to both; from D arises the hydrotheca (E) or the gonotheca (F); FIG. 13.