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An early or simple larval stage of trematode worms and some other invertebrates, which is capable or reproducing other germs by asexual generation; a nurse; a redia.

or cartilaginous plates or segments, in the median line of the pectoral skeleton of most vertebrates above fishes; the breastbone.

special ganglion situated beneath the esophagus of arthropods, annelids, and some other invertebrates.

n. -- Sucking stomach (Zoöl.), the muscular first stomach of certain insects and other invertebrates which suck liquid food.

s (Anat.), two small bodies of unknown function in front of, or near, the kidneys in most vertebrates. Also called renal capsules, and suprarenal bodies.

ilages representing them) which connects the base of the lower jaw with the skull in most vertebrates below mammals.

onnected with the alimentary canal, the vascular system, and the glandular organs of most vertebrates, and controlling more or less their actions. The axial part of the system and its principal ganglions and nerves are situated in the body cavity and form a chain of ganglions on each side of the vertebral column connec…

d squamosal portion above the ear, corresponding to the squamosal and zygoma of the lower vertebrates, and a thickened basal petrosal and mastoid portion, corresponding to the periotic and tympanic bones of the lower vertebrates.

ed process or organ, simple or branched, proceeding from the head or cephalic region of invertebrate animals, being either an organ of sense, prehension, or motion. Tentacle sheath (Zoöl.), a sheathlike structure around the base of the tentacles of many mollusks.

The external hard or firm covering of many invertebrate animals.

Invertebrate animals covered with shells, especially mollusks; shellfish.

Thymus body, a ductless gland in the throat, or in the neighboring region, of nearly all vertebrates. In man and other mammals it is the throat, or neck, sweetbread, which lies in the upper part of the thorax and lower part of the throat. It is largest in fetal and early life, and disappears or becomes rudimentary in…

an organ situated in the floor of the mouth of most vertebrates and connected with the hyoid arch.

ich are borne on the jaws, or on other bones in the walls of the mouth or pharynx of most vertebrates, and which usually aid in the prehension and mastication of food.

A grand division of the animal kingdom, intermediate, in some respects, between the invertebrates and vertebrates, and by some writers united with the latter. They were formerly classed with acephalous mollusks. The body is usually covered with a firm external tunic, consisting in part of cellulose, and having two open…

, a system of organs which eliminate nitrogenous waste matter from the blood of certain invertebrates.

A receptacle, or pouch, connected with the oviducts of many invertebrates in which the eggs are retained until they hatch or until the embryos develop more or less. See Illust. of Hermaphrodite in Append.

The terminal part of the oviduct in insects and various other invertebrates. See Illust., of Spermatheca.

er vascular system (Zoöl.), a system of vessels in annelids, nemerteans, and many other invertebrates, containing a circulating fluid analogous to blood, but not of the same composition. In annelids the fluid which they contain is usually red, but in some it is green, in others yellow, or whitish.

The anal opening of certain invertebrates and fishes; also, the external cloacal opening of reptiles, birds, amphibians, and many fishes.