## e-h

### e

- echinoderm n. Phylum Echinodermata. A group of marine invertebrate animals, including sea urchins, sea stars, and sea cucumbers; characterized by a skeleton made of little plates, which may be a rigid armor as in sea urchins, or made of small plates in a leathery body wall as in holothurians (sea cucumbers).
- electrode n. A conductor that represents the positive or negative pole of an electric circuit or battery.
- embryo n. The developmental stages while constrained in an egg or parent.
- embryology n. The study of the formation and development of an embryo and fetus.
- ephyra n. The juvenile form of a medusa that buds off the polyp stage of a jellyfish and may be produced in groups that resemble tiny stacks of lifesavers without the hole.
- eurypterid n. Phylum Arthropoda, Subphylum Chelicerata, Class Meristomata, Order Eurypteridae A type of large, predatory, extinct marine arthropod that flourished during the Silurian Period (438 to 408 million years ago). See sea scorpion.
- evolution n. The process whereby new species arise typically as a result of gradual changes that occur in populations or organisms over a long period of time.
- exoskeleton n. A hard external covering of various animals, including arthropods, that provides attachment for muscles and protects the animal from drying out and/or mechanical injury.

### f

- feather duster worm n. Phylum Annelida, Class Polychaeta. Any large, segmented, marine tube worm of the family Sabellidae that typically exposes only a showy crown of feeding tentacles and keeps the rest of the body tucked away out of sight.
- flatworms n. Phylum Platyhelminthes A group of soft-bodied, usually much-flattened invertebrates that are free-living as well as parasitic; bilaterally symmetrical (i.e., the right and left sides are similar); and lack respiratory, skeletal, and circulatory systems as well as a body cavity.
- forelimb n. The front limb of an animal -- usually a tetrapod (a vertebrate with four appendages).
- fossil n. the impression or trace of an ancient organism in a rock or the remains of an organism that has been chemically preserved.
- frequency n. The number of cycles undergone during one unit of time.
- funnel n. A modification of the mollusc foot in such cephalopods as squid and octopus that aids in jet propulsion.

### g

- gene n. The unit of hereditary information consisting of a specific sequence of DNA or RNA (for viruses) that codes for a protein.Gene sequencing The process to determine the order of nucleotide bases in a section of DNA.
- genetic code n. The triplets of nucleotides that specify which amino acids occur sequentially along a gene.
- genetics n. The study of heredity in general and of genes in particular.
- geologic record n. Information captured in rock and decipherable by geologists.
- gill bars n. Structural skeletal elements that support the gas exchange organs of various aquatic animals, like fish and amphibians.
- gill slits n. Pouches or slits in the front region (pharynx) of the digestive tract in chordates. These function in feeding and gas exchange in fishes and some amphibians, but appear during development in all vertebrates.

### h

- head n. A distinct region of the front end (anterior) of an animal, containing the brain and typically the mouth and sense organs.
- hermaphrodite n. An organism that produces both female and male sex cells.
- horseshoe crab n. Phylum Arthropoda, Order Xiphosura, Class Merostomata. A marine animal found on the east coasts of Asia and North America, related to the scorpions and spiders.
- hot vent n. fissures in the deep ocean that spew very hot water filled with chemicals that bacteria live on. A whole community of organisms is supported by these bacteria.
- hox genes n. Genes that control the overall body plan of organisms by controlling other genes and the developmental fate of groups of cells.