GLOSSARY 



509 



Swim-bladder, or Air-bladder, a gas-contain- 

 ing- outgrowth from the digestive tube of 

 some Fishes, which serves as a hydrostatic 

 organ. 



Symbiosis (Gk. syn, together; bidsis, life). See 

 Mutualism. 



Symmetry (the Greek name), regularity of build. 

 In Radial Symmetry the parts of the body are 

 arranged with reference to a centre of sym- 

 metry, e.g. in a coral polype. In Bilateral 

 Symmetry, as seen, e.g., in a Fish, we can 

 distinguish right and left sides, anterior and 

 posterior ends, upper and lower surfaces. 



Sympathetic nervous system, a part of the 

 nervous system concerned with the regula- 

 tion of the blood-vessels and internal organs. 



Syndactylous (Gk. syn, together; ddktyltis, a 

 finger or toe), with some of the digits bound 

 closely together. 



Syrinx (Gk. for reed-pipe), the vocal organ or 

 voice-box of a Bird, situated where the wind- 

 pipe divides into a branch (bronchus) for each 

 lung. 



Systemic, applied to a heart which contains 

 pure blood only. 



Tail-coverts, feathers covering the tail-quills. 

 Tarsale, pi. -ia, one of the distal elements of 



the tarsus. 



Tarso -metatarsus, a bone in the leg of a Bird, 

 formed by the fusion of part of the ankle- 

 skeleton (tarsus) with three of the instep- 

 bones (metatarsus). 



Tarsus (Gk. tarstis, the broadest part of the 

 foot): (i) in Vertebrates, the skeleton of the 

 ankle ; (2) in Insects, the foot. 

 Teeth : (i) in Vertebrates, hard structures used 

 for securing prey, or for breaking up food ; 

 ( ) hard projections with which the radula 

 (which see) of Molluscs is studded ; (3) small 

 projections usually present in the "hinge" 

 of a bivalve shell : they fit into correspond- 

 ing sockets, and prevent shifting when the 

 shell opens or closes. 



Telson (Gk. for a termination), in higher 

 Crustaceans, the last segment of the abdo- 

 men. It bears no limbs, and is the middle 

 part of the tail-fin. 



Tendon, a fibrous inelastic band by which a 

 muscle is attached to a part of a skeleton. 

 Tensor muscles (L. tendo, tentum, tensum, to 

 stretch), in Birds, muscles which keep the 

 wing-membranes on the stretch. 

 Tentacle (L. tento, I feel), a soft feeler, e.g. 

 one of the " horns " of a Snail, or one of the 

 numerous fleshy filaments surrounding the 

 mouth-end of a Sea-Anemone. 

 Tentaculocyst (tentacle; Gk. cystts, a bladder), 

 in some Jelly-Fishes, a short specialized ten- 

 tacle serving as a balancing organ. 

 Tergum, pi. -a (L. for the back), in Arthro- 

 pods, that part of the exoskeleton covering 

 the dorsal surface of a segment. 

 Test (L. testa, a shell), the firm investment of 

 Ascidians, Echinoderms, and some other 

 lower animals. 



Tetradactyle(Gk. tetra, four; daktyltfs, a finger 



or toe), possessing four digits. 

 Thoracic duct, a narrow tube lying imme- 

 diately ventral to the backbone. It receives 

 most of the lymphatics, and opens into the 

 great veins at the base of the neck. 

 Thread-cell. See Nettling-cell. 

 Thymus gland (Gk. thymtis, the heart), a 

 fatty - looking ductless gland situated (in 

 Mammals) near the base of the heart. 

 Thyroid gland (Gk. thyretis, a shield; eidtis, 

 resemblance), a ductless gland in the neck- 

 region, having something to do with regu- 

 lating the nutrition of the body. 

 Tibia (the Latin name for(i)): (i) the shin-bone 

 of Vertebrates ; (2) that region of the leg in 

 Insects, &c., which adjoins the foot. 

 Tibiale, in the tarsus, a proximal element situ- 

 ated on the side next the great toe. 

 Tibio -tarsus, in Birds, a bone formed by fusion 

 of the shin-bone (tibia) with part of the ankle- 

 skeleton (tarsus). 



Tissue, an aggregate of cells specialized for 

 the performance of some particular kind of 

 physiological work. 



Tornaria, pi. -ae (Gk. ttfrntfo, I make round), 

 the ciliated larva of some species of acorn- 

 headed worm. 



Tortoise-shell, an ornamental product pre- 

 pared from the horny epidermic plates of 

 certain Turtles. 

 Trachea (Gk. tracheliaitis, relating to the 



neck), the wind-pipe. 

 Tracheae. See Trachea! tubes. 

 Tracheal gill, in some aquatic Insects, a gill 



traversed by air-tubes (trachece). 

 Tracheal tubes, the air-tubes of air-breathing 



Arthropods. 

 Tragus, a pointed projection (" earlet ") within 



the ear of a true Bat. 

 Trepang, dried Sea-Cucumbers, an important 



article of food in the Far East. 

 Trez-tine. See Antler-royal. 

 Trichinosis, a disease due to the attacks of 



minute thread-worms (Trichinae). 

 Trichocysts (Gk. thrix, trichtis, a hair; cystis, 

 a bladder), in some Animalcules, micro- 

 scopic rods discharged from the outer layer 

 of the body as a means of defence, and pro- 

 bably of irritant nature. 

 Tridactyle (Gk. tri, three; daktylos, a finger 



or toe), possessing three digits. 

 Triploblastic (Gk. triples, threefold; blastos, 

 a bud), applied to animals in which the body 

 essentially consists of three cellular layers. 

 Trochanter (Gk. for a process of the thigh- 

 bone), in Insects, the small second joint of 

 the leg. 



Trochosphere (Gk. trochos, a wheel; sphaira, 

 a sphere), in various Invertebrates, a bilateral 

 ovoid larva, with a well-marked head-lobe, at 

 the base of which is a circlet of large cilia. 

 Tubercle, a rounded projection, e.g. on the 

 crown of a tooth. 



