Gastropoda: A class in the phylum MOLLUSCA comprised of SNAILS and slugs. The former have coiled external shells and the latter usually lack shells.Snails: Marine, freshwater, or terrestrial mollusks of the class Gastropoda. Most have an enclosing spiral shell, and several genera harbor parasites pathogenic to man.Mollusca: A phylum of the kingdom Metazoa. Mollusca have soft, unsegmented bodies with an anterior head, a dorsal visceral mass, and a ventral foot. Most are encased in a protective calcareous shell. It includes the classes GASTROPODA; BIVALVIA; CEPHALOPODA; Aplacophora; Scaphopoda; Polyplacophora; and Monoplacophora.Biomphalaria: A genus of planorbid freshwater snails, species of which are intermediate hosts of Schistosoma mansoni.Lymnaea: A genus of dextrally coiled freshwater snails that includes some species of importance as intermediate hosts of parasitic flukes.Nematocyst: A double-walled capsule found in jellyfish and other CNIDARIA whose functions include prey capture, defense, locomotion, and attachment. Nematocysts contain toxic CNIDARIAN VENOMS which are injected into the victim via a barbed tubule.Fasciola: A genus of trematode liver flukes of the family Fasciolidae. Two species of this genus are F. hepatica and F. gigantica. The parasites are found in the liver and gallbladder and associated ducts in mammals and occasionally man. F. gigantica occurs rarely in man.RNA, Ribosomal, 18S: Constituent of the 40S subunit of eukaryotic ribosomes. 18S rRNA is involved in the initiation of polypeptide synthesis in eukaryotes.Disease Vectors: Invertebrates or non-human vertebrates which transmit infective organisms from one host to another.Phylogeny: The relationships of groups of organisms as reflected by their genetic makeup.Population Density: Number of individuals in a population relative to space.BrazilEcosystem: A functional system which includes the organisms of a natural community together with their environment. (McGraw Hill Dictionary of Scientific and Technical Terms, 4th ed)Seasons: Divisions of the year according to some regularly recurrent phenomena usually astronomical or climatic. (From McGraw-Hill Dictionary of Scientific and Technical Terms, 6th ed)DNA, Ribosomal: DNA sequences encoding RIBOSOMAL RNA and the segments of DNA separating the individual ribosomal RNA genes, referred to as RIBOSOMAL SPACER DNA.Reproduction: The total process by which organisms produce offspring. (Stedman, 25th ed)Sequence Analysis, DNA: A multistage process that includes cloning, physical mapping, subcloning, determination of the DNA SEQUENCE, and information analysis.