TEN 



[254] 



T E R 



of a great portion of the existing species. 

 Mr. Lyell. 



TENA'CITY. (tenacitas, Lat. tenacite, Fr.) 

 Viscocity ; adhesiveness ; glutinousness ; 

 cohesiveness. The degree of force with 

 which the particles of bodies cohere.- It 

 is in consequence of possessing the pro- 

 perty of tenacity, that certain substances 

 permit themselves to be drawn out into 

 wire or flattened under the blows of the 

 hammer. 



TE'NDON. (tendo, Lat. from rtiva), to 

 stretch, Gr. tendon, Fr.) The fibrous 

 cord-like extremity of a muscle. 



TE'NDINOUS. (tendineux, Fr. tendinoso, 

 It.) Composed of tendons ; resembling 

 tendon. 



TE'NDRIL. (tendron, Fr.) In botany, a 

 spiral appendage to certain plants, its 

 use being to clasp and wind round other 

 bodies, by which means weak and climb- 

 ing stems support themselves, and rise to 

 a great height. 



TE'NNANTITE. A variety of sulphuret of 

 copper of a lead-grey or blackish colour. 

 It has been thus named by Mr. R. 

 Phillips. It occurs in copper veins in 

 some of the mines of Cornwall. Its 

 constituents are, according to the analysis 

 of Mr. Phillips, copper 45'32, arsenic 

 11-84, iron 9'26, sulphur 28'74, silica 

 5'00. Hardness 4*0. Specific gravity 

 from 4'3 to 4-4. It occurs massive, and 

 crystallized in rhomboidal dodecahedrons, 

 cubes, and octohedrons. 



TE'NTACLES. ) Feelers ; exploring organs. 



TENTA'CULA. ) In its most restricted 

 sense, says the Rev. W. Kirby, this term is 

 understood to signify organs, appendages 

 of the mouth, which have no articula- 

 tions, but, in a larger sense, the term has 

 been applied also to all jointed organs in 

 its vicinity, and used for a similar pur- 

 pose, which indeed are the precursors of 

 feelers and antennae. It is to these or- 

 gans, as well as for their food, that 

 polypes are indebted for what consti- 

 tutes their principal ornament, that re- 

 semblance of a plant or shrub in full 

 blossom adorned with crimson or orange- 

 coloured flowers. In the fixed polypes, 

 the tentacles are the only motive organs. 

 The tentacles of the fresh-water polypes, 

 forming the locomotive genus hydra, are 

 not, as those of the fixed marine ones, 

 shaped like the petals of a blossom, but 

 are long hair-like flexile arms, somewhat 

 resembling the branches of a chandelier. 

 Amongst the Radiaries, tentacles exist in 

 some genera, and not in others. In the 

 Stelleridans and Echinidans, there are 

 no tentacles, but the Fistulidans present 

 a floriform coronet of tentacles. Ten- 

 tacles as exploratory, prehensory, and 

 locomotive organs, exist in several other 



classes of animals. In none, however, 

 are they more remarkable than in the 

 Cephalopoda : in these animals they are 

 used as arms for prehension, as legs for 

 locomotion, as sails for wafting their pos- 

 sessors over the boundless deep, as oars 

 for passing through its waves, as a rudder 

 for directing their course, and as an an- 

 chor for fixing themselves. The tenta- 

 cula of the various tribes of Polypi, of 

 Actiniae, and of Annelida, are organs both 

 of prehension and of touch. 



TEREBE'LLA. A genus of Annelidans, or 

 annulose animals, placed by Cuvier in 

 the order Tubicola. They are inhabi- 

 tants of the sea, and are met with 

 generally in shallow water, on the coasts, 

 and on shells, &c. The body is oblong, 

 creeping, naked, often enclosed in a tube, 

 furnished with lateral fascicles, or tufts, 

 and small branchiae ; mouth placed be- 

 fore, furnished with lips without teeth, 

 and protruding a clavate proboscis ; 

 feelers numerous, ciliate, capillary, seated 

 round the mouth. Terebellae not being 

 provided by nature with any external 

 shell, endeavour to furnish themselves 

 with an armature. For this purpose they 

 collect grains of sand, or fragments of 

 decayed shells, or other substances, which 

 they agglutinate together by means of a 

 viscid exudation, so as to form a firm 

 defensive covering, like a coat of mail. 

 These coverings, however, composed as 

 they are of extraneous materials, and 

 not being organic productions of the ani- 

 mals themselves, are structures wholly 

 foreign to their systems. Terebellae are 

 provided with tentacles, issuing from the 

 head, which, when the rest of the body 

 has retired within the tube, is the only 

 part exposed. 



TE'REBRA. A genus of turreted subulated 

 marine univalves : the opening short, and 

 notched in the lower part. The basis of 

 the columella twisted. Two species are 

 found fossil in the environs of Paris, 

 namely, Terebra plicatula, and Terebra 

 scalarina. 



TE'REBRATING. A term applied to shells 

 which form holes in rocks, wood, &c., 

 and reside therein. 



TEREBRA'TULA. A genus of the class 

 Brachiopoda. Terebratulse are marine 

 bivalves found moored to rocks, shells, 

 &c., at depths varying from ten to ninety 

 fathoms. The valves are unequal and 

 united with a hinge, but having no liga- 

 ment : the summit of one, more salient 

 than the other, is perforated to permit 

 the passage of a fleshy pedicle, by means 

 of which the animal attaches itself to 

 rocks, shells, &c. The recent species are 

 few, but the fossil are very numerous. In 

 the fossil shell, the operculum which 



