![]() Above the base is a ring of contractile cells called the sphincter. Often there will also be connections here to hollow tubes (called stolons) that connect the polyp to others in its colony, and allow the exchange of food between polyps. At their base hydrozoan polyps have basal disks or elongate processes for attaching to substrate, or they may be attached to other polyps. In most species they are only a few millimeters tall, though the largest grow up to many centimeters, and one, Branchiocerianthus imperator can be 2 meters tall. Polyps are radially symmetric, and may be urn-shaped, conical, cylindrical, or club-shaped. Planulae most often settle onto a benthic substrate and develop into a polyp. They may be very simple embryos or have cells differentiated into several types. Planulae are radially symmetric ovoids, often covered with flagellate cells for swimming. Most hydrozoan species have a planktonic larval stage called a planula. The small number of freshwater species occur in both lotic and lentic habitats, and are more abundant in eutrophic and mesotrophic waters. They are most abundant and diverse in warm shallow waters, probably as a reflection of food abundance. Hydrozoans are found in nearly all marine habitats, except perhaps heavy surf zones. A few species occur in fresh and brackish water on all continents except Antarctica. Hydrozoans are found in all the oceans, at all latitudes. ( Bouillon, et al., 2006 Brusca and Brusca, 2003 Jankowski, et al., 2008 Mills, 2009) Geographic Range Most hydrozoans are predators or filter-feeders, though a few have symbiotic algae (zooxanthellae), in the same way that other other groups of cnidarians do.īetter-known hydrozoans include Portuguese man-o-wars ( Physalia physalis), the freshwater genus Hydra, fire coral ( Milleporidae), and by-the-wind sailors ( Velella velella). Colonial polyps often have some division of function, with certain polyps specialized for defense, feeding, or reproduction. Colonies are usually benthic, but some, notably the siphonophores, are pelagic floaters. Most hydrozoans form colonies of asexual polyps and free-swimming sexual medusae. Most hydrozoans are marine, and hydrozoan species are found in nearly every marine habitat type a very few species live in freshwater. ![]() They are distinguished from other groups by their complex life cycle, by the growth of medusae from buds rather than strobilae or from metamorphosis, by the presence of a velum inside the bell of the medusa, and by the production of gametes from ectodermal, rather than endodermal, tissue. Like many cnidarians, hydrozoans have both polyp and medusa stages in their life cycle. It is a diverse group with a variety of life cycles, growth forms, and specialized structures. In this paper we (a) show the syntax and semantics of the C-SPARQL language together with some examples (b) introduce a query graph model which is an intermediate representation of queries devoted to optimization (c) discuss the features of an execution environment that leverages existing technologies (d) introduce optimizations in terms of rewriting rules applied to the query graph model, so as to efficiently exploit the execution environment and (e) show evidence of the effectiveness of our optimizations on a prototype of execution environment.The Hydrozoa is a subgroup of cnidarians containging approximately 3700 species. Executing C-SPARQL queries requires the effective integration of SPARQL and streaming technologies, which capitalize over a decade of research and development such integration poses several nontrivial challenges. In this domain, sensor-based information on roads must be processed to deduce localized traffic conditions and then produce traffic management strategies. It covers a gap in the Semantic Web abstractions which is needed for many emerging applications, including our focus on Urban Computing. Continuous SPARQL (C-SPARQL) is proposed as new language for continuous queries over streams of RDF data.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |