Statocyst. Organ of balance consisting of a vesicle, containing granules of lime, sand, etc. (stotoliths) which stimulate sensory cells as the animal moves; present in some Crustacea, flatworms, etc. Some time known as otocyst and otoliths.

Stearic acid. A saturated fatty acid having 18 carbon atoms which occurs in animal fat.

Stele. (Vascular cylinder). Cylinder or core of vascular tissue in centre of roots and stems, consisting of xylem, phloem, pericycle, and, in some steles, pith and medullary rays; surrounded by endodermis. Structure of the stele differs in different groups of plant. (see Dictyostele, Protostele, Siphonostele, Vascular Bundle.)

Stenohaline. Unable to tolerate wide variation of osmotic pressure of environment.

Sternite. The skeletal plate(s) on the ventral side of insect and cruslaceam body segments.

Sternum.
(1) Breast bone. Bone of terrestrial vertebrates (tetrapods) in the middle of the ventral side of the chest, to which the ventral ends of most of the ribs are attached. At its anterior end attached to shoulder gridle.

(2) Of arthropods, cuticle on ventral side of each segment, often forming a thickened plate.

Stipel. One of two leaf-like structures at the base of a leaflet in some compounds leaves.

Stomach. Enlargement of the anterior region of the gut. In vertebrates follows oesophagus, and usually has muscular walls which churn food, and lining cells which secrete pepsin and hydrochloric acid. Initial steps in digestion occur there, food being held by pylorus until reduced to a mush.

Stomium. Place in the wall of fern sporangium where rupture occurs at maturity, releasing the spores.

Strain. A group of organisms within a species or variety, distinguished by one or more minor characteristics.