a type of cleavage pattern found in deuterostomes (echinoderms, chordates) in which the daughter cells cleave off without rotation. Contrast with spiral cleavage of protostomes (annelids, molluscs, arthropods)
Radial symmetry:
see Symmetry
Radula ("scraper"):
a specialized toothed device used for feeding in molluscs (not found in Bivalvia)
Recapitulation ("to repeat"):
Respiratory tree ("respiratio": breathing ):
unique paired and branched structures originating from the cloaca of sea cucumbers, suspended in the coelom and involved in gas exchange
Rhabdites or rhabdoids ("rod"):
these are rod-like organelles found in some flatworms (e.g., Planaria) produced either by gland cells in the mesenchyme (in which case they are known as rhabdites) or by epidermal cells in the skin (rhabdoids). When released they rupture and create mucus which may function in defense or perhaps in locomotion
Rhopalium ("club"):
a specialized sensory device found in sea jellies (scyphozoans) combining light- and balance-perception. There are usually 8 of these arranged symmetrically around the margin of the bell
Scaphopoda ("hatchet" "foot"):
a small Class of Phylum Mollusca known as tusk shells
Schizocoely:
a theory of coelom origin in which splits appear in the mesoderm which later enlarge. This type of coelom development is seen in protostomes (annelids, molluscs, arthropods). Contrast with enterocoely as found in deuterostomes (echinoderms, chordates)
Sclerite ("scleros": hard):
hard plates, as part of the arthropod cuticle. Axillary sclerites are found at the base of the wings in some or all Insecta, and their articulation allows the wings to be folded back (not in primitive insects such as dragonflies) and their position in flight adjusted during wing-beats
Sclerocyte ("hard" "cell"):
specialized cells that produce the spicules in sponges
Sclerotize ("hard" "to make "):
the hardening of protein by creation of covalent cross-linking between adjacent polypeptide chains. Phenols are involved in the process. In chitin the cross-linkings strengthen the polymer chains and create a tough, not easily broken down or digestible, substance
Scolex ("worm" or "grub"):
the unique anterior attachment organ of tapeworms, endowed with suckers and often hooks. Proglottids develop from behind the scolex
Scutum ("shield"):
the ventral protective plates in uniramians. These plates articulate with one other by means of soft, flexible membranes. They articulate with the dorsal tergal plates by means of a similar flexible membrane, the pleuron
Scyphistoma ("cup" "mouth"):
an asexually reproducing polyp stage in scyphozoans (jellyfish) that buds off juvenile medusae known as ephyrae in a process known as strobilation
Scyphomedusa ("cup" "Medusa: creature in Gr mythology"):
the sexually-reproducing medusa stage of scyphozoans (jellyfish). Medusa was one of the 3 Gorgons in Greek mythology. She apparently had snakes for hair and turned anyone who looked at her into stone
Scyphozoa ("cup" "animal"):
a Class of Phylum Cnidaria which includes jellyfish
Segmentation:
where the body is differentiated along the longitudinal axis into a series of units or segments, each containiing parts of organ systems. In an "idealized" form, each segment contains: muscle, nephridia, nerves, blood vessels, external appendages, and a portion of the gut
Seminal receptacle ("seed" "container"):
part of the female reproductive system where sperm received from a male during mating is stored for later use to fertilize the eggs
Seminal vesicle ("seed" "bladder"):
part of the male reproductive system where sperm is stored prior to mating
Septum ("wall"):
a sheet of tissue that separates two compartments or cavities, as in annelids
Seta:
a bristle as on annelids or arthropods
Siphonoglyph ("tube or pipe" "carving or engraving"):
in sea anemones these are symmetrically paired openings on either side of the mouth which are apparently used for passage of water when the mouth is closed. Some anemones have only a single siphonoglyph
Size-strength ratio:
relative strength in a tubular system, as that of a flying insect, is greatest when diameters are small. Hence, smaller insects are relatively stronger than larger ones
Spermatheca ("seed" "case or cover"):
part of the female reproductive system in insects where sperm received from the male during mating is stored for later use in fertilizing the eggs (see also seminal receptacle)
Spermatophore ("seed" "to carry"):
a packet of sperm passed to the female or inserted into the femal reproductive tract by the male (as in cephalopods, spiders, crustaceans)
Spicule ("small spike"):
any needle-like structure protective or skeletal structure found in invertebrates; in sponges, the needle-like structures produced by sclerocytes used for support and protection
Spiracles: ("air hole"):
outside openings of tracheal or tracheal-like gas-exchange systems found in arthropods such as insects, myriapods, a few isopods, and in onychophorans. Only in insects can the spiracles be closed
Spiral cleavage:
one of two main types of cleavage patterns in animals in which successive division products shift back and forth, clockwise and counterclockwise. Spiral cleavage is found in the group of animals known as protostomes: annelids, arthropods, molluscs
Spongin:
a resiliant protein fibre that makes up the "spongy" material in demosponges
Spongocoel ("sponge" "hollow"):
the space within a sponge that receives the exhalent water after it exits from the flagellated (choanocyte) chambers. Because sponges do not gastrulate, there is no endoderm and, thus, no gastrodermis and, thus, this space is NOT a gut
Statocyst ("standing" "bag"):
a sensory device providing information on balance. It usually consists of a small space lined by sensory cells and containing a solid concretion (as a sand grain or bit of calcium carbonate)
Stellate ganglion ("star"):
a large ganglionic cluster embedded on either side of the mantle wall in cephalopods with nerves radiating to all parts of the mantle. The stellate ganglia are involved in jet-propulsive locomotion. Nerves from the stellate ganglia increase in diameter with length; thus, since speed of nerve impulses is directly related to diameter of the neurones, impulses tend to arrive at all parts of the mantle simultaneously. This increases the effectiveness of the jet in locomotion
Sternum ("breast or chest"):
in arthropods, the plate on the ventral surface of each body segment
Stolon ("shoot" or "branch"):
in colonial tunicates and cnidarians, a tissue connection between zooids or polyps allowing transfer of nutrients and energy
Strobila ("twisted" or a "cone"):
the stage in the life cycle of a scyphozoan also known as an ephyra. Strobilation is the process of asexual division of the scyphistoma polyp into ephyrae
Suspension feeding
a mode of feeding where particulate food matter suspended in the surrounding water is removed; also known as filter-feeding
Sycon:
an intermediate morphological type of sponge with the flagellated chambers occupying inpocketings off the spongocoel
Symbiosis ("to live together"):
a general term for two organisms living together. Mutualism, commensalism, parasitism, and inquilinism are all types of symbioses
Symmetry:
is the arrangement of parts into geometrical designs. Invertebrates have spherical, radial, biradial, or bilateral symmetry. Spherical symmetry is obvious and is found in a few groups of protists to one degree or another. Radial symmetry is found in hydroid polyps or sea jellies, which have a single longitudinal axis about which the body parts are arranged. Here, the body can be divided into an infinite number of symmetrical halves by single hypothetical cuts along the longitudinal axis. If some structures are off-centre, such as siphonoglyphs in a sea anemone, then the symmetry is biradial because symmetrical halves can be produced in only two ways: by a cut down the longitudinal and transverse axes, and by a cut down the longitudinal and sagittal axes. Any other cut produces asymmetrical halves. Note: 1) the designation of transverse and sagittal axes is arbitrary (one can be the other) and 2) the off-centre structures are themselves arranged symmetrically. If structures are off-centre and not arranged symmetrically, then like halves can be produced by only a single cut through the longitudinal and sagittal axes. This type of symmetry, bilateral, is a characteristic of all "higher" animals.
Symplesiomorphy("shared" "ancestral" "character'):
a shared ancestral character state as inferred from the most parsimonious distribution of character states on a specific cladogram. Contrast with synapomorphy
Synchrony-asynchrony in wing-beats:
this refers to whether wing-beats in insects are in synchrony or not with nervous stimulation of the flight muscles. If muscles attach directly to the wing bases, then the wing-beat is likely to be synchronous; that is, elevator and depressor muscles will separately be stimulated by a nerve impulse (or set of impulses), each stimulus causing the wing to move up or down. This is a primitive condition, only found in odonatans (dragonflies, mayflies). Most insects have a combination of muscle/nerve innervation and attachments. Midges, some of which can beat their wings up to 1000 times per second, have an asynchronous/indirect system which allows a certain number of oscillations (wingbeats) to occur from a single nerve impulse; otherwise, there would not be time enough to regenerate action potentials sufficiently fast enough to run the system
Synapomorphy ("shared" "derived" "character'):
a shared derived character state as inferred from the most parsimonious distribution of character states on a specific cladogram. Contrast with symplesiomorphy
the larval type of tunicates (sea squirts). Only the tadpole larva has the 4 chordate characteristics of notochord, dorsal tubular nerve cord, pharyngeal gill slits, and tail. All but the gill slits are lost during metamorphosis to the adult sea squirt. Tadpole larvaae function mainly in substrate selection and not so much in despersion, as their free-living lives are generally quite short
Tanning ("tanning"):
a process in which fibrous proteins are strengthened by chemical cross-linking, usually involving the formation of bonds between sulphur-containing amino acids in adjacent protein strands
Tegument ("to cover"):
the protective outer covering of parasitic flatworms, including flukes and tapeworms, consisting of a syncytial (cell nuclei not separated by cell boundaries; hence, multinucleate protoplasm) outer layer connected to cells lying deep within the worm's mesenchyme
Tergum ("back"):
the dorsal or uppermost exoskeletal plate in uniramians such as insects, millipedes, and centipedes. The tergal plates articulate with one another by means of soft, flexible membranes and with the ventral scutal plates by means of a similar flexible membrane, the pleuron
Test ("shell"):
usually an outer, nonliving shell or case surrounding an organism; in echinoids (sea urchins), however, the test is actually an endoskeleton produced from mesoderm
Tetrodotoxin:
a neurotoxic poison produced in the anterior salivary glands of cephalopods used in conjuction with the beak for immobilizing and killing prey
Torsion ("twist"):
a twisting of the head/velum 180 degrees on the foot during early larval life in gastropods. This brings the mantle cavity to the front and creates a problem of fouling of the head with anal and kidney discharges, but provides a protective space represented by the mantle cavity into which the head/velum and foot can be withdrawn. A protective plate or operculum attached to the foot then plugs the hole
Totipotent ("whole or all" "potency"):
with reference to a cell: one that can change into any one of a variety of specialized cells in the organism (e.g., amoebocyte of a sponge or lymphocyte of a tunicate)
Trachea ("trough"):
refers in invertebrates to the branching system of air-tubes found in arthropods such as insects, spiders, and myriapods (centipedes, millipedes), in onychophorans and in some terrestrial isopods. At their finest diameter, the tubes are known as tracheoles, are fluid-filled, and penetrate into the tissue cells to allow direct exchange of respiratory gases
Trematoda:
a Class of parasitic Platyhelminthes known as liver flukes
Triaxon ("three" "axis"):
a type of sponge spicule with 3 axes. Since this type of spicule has 6 rays, it is also known as hexactinellid, and is the spicule type characterizing the Class of sponges known as Hexactinellida
Trichinosis ("hair"):
the state of being parasitized by the nematode Trichinella spiralis
Tricladida ("three" "branch"):
an Order of turbellarian flatworms characterized by having a 3-branched gut; e.g., Planaria. Triclads are freshwater, marine, and terrestrial
Trilobitomorpha:
a SubPhylum of Phylum Arthropoda which includes the extinct trilobites
Triploblastic ("three" "buds"):
refers to the 3 cell-layer state of animals which have endoderm (syn. entoderm), ectoderm, and mesoderm. All bilaterian animals are triploblastic (cnidarians are diploblastic)
Trochophore larva ("wheel" "bearing"):
primitive larval type of marine molluscs characterized by a circlet of propulsive and food-gathering cilia. This stage precedes the veliger larval stage in classes of molluscs where the latter is present (scaphopods, gastropods, bivalves). A trochophore larva is also present in polychaetes, thus suggesting homologous origins of molluscs and annelids
Tube foot
fluid-filled tubular structures that are part of the water-vascular system of echinoderms. These soft hydraulic structures can be extended and retracted, and function in feeding, locomotion, anchoring, and gas exchange
Tunicin ("tunic or cloak"):
a type of exoskeleton functioning in support and protection in tunicates (sea squirts), consisting of a combination of protein and mucopolysaccharides (tough and indigestible)
Turbellaria:
a Class of free-living Platyhelminthes, including colorful marine forms and the freshwater-inhabiting Planaria
Tympanum ("drum"):
a taut membrane located at the junction of the thorax and abdomen in insects, used for sound perception
Typhlosole ("blind" "pipe"):
a lobulated ridge of tissue extending into the lumen of the gut of earthworms which presumably functions to increase the digestive and absorptive surface area of the gut
Uniramia ("one" "branch"):
a SubPhylum of Phylum Arthropoda which includes the millipedes, centipedes, and insects. The name uniramia refers to the fact that the appendages are unbranched, or uniramous
Uric acid:
an excretory chemical containing amine groups (NH2) used in insects and terrestrial gastropods as a means to rid the body of unwanted ammonia produced as a byproduct of metabolism. Uric acid is insoluble and in insects is excreted as harmless crystals which are voided from the body along with the feces
Urochordata:
a SubPhylum of Phylum Chordata which includes the sea squirts, mucus-house builders, and appendicularians
Veliger larva ("veil or sail"):
advanced larval type of marine molluscs characterized by a ciliated lobe (or lobes) known as the velum which functions in propulsion and food-gathering
Velum ("veil" or "covering"):
in hydromedusae is a thin flap of tissue around the inner surface of the bell, perhaps functioning venturi-like to increase the velocity of the locomotory jet of water; in the veliger larva of molluscs is a double-lobed ciliated structure used to locomote and filter-feed
Vermiform ("wormlike"):
wormlike in appearance; in locusts, the egg hatches to a "vermiform larva" that wiggles its way to the soil surface - it then splits out of the vermiform cuticle and becomes the 1st instar stage
Visceral mass ("viscera": internal organs):
one of the 3 main parts of a mollucan body: head, foot, and visceral mass. It is the fleshy part that contains the main organ systems
Volvent ("roll or turn"):
a type of nematocyst found in Hydra which coils tightly on discharge, thus tangling a prey. Volvents are non-toxic and, owing to their special function, have a low discharge threshold on the chemical (prey-perception) side
system of water ducts and tubefeet in echinoderms (seastars, sea urchins, sea cucumbers) used in locomotion, anchoring, gas exchange, and feeding
Zoea larva("animal" "larva"):
the first free-living larval stage of crabs which transforms into a crab-like megalops stage prior to settling from the plankton. Zoeae are carnivorous, feeding on other zooplankters