220 INVERTEBRATE ZOOLOGY 
Sclerite: a small plate forming a portion of the cuticula of a segment 
in insects. 
Scolex: the anterior end of a tapeworm, 80. 
Scutellum: a small sclerite in the tergum of the thoracic segments in 
insects. 
Segment: one of a number of serial divisions of an animal’s body or of 
an organ. 
Septum: a plate forming a division wall between two spaces; Nereis, 63; 
earthworm, 69; mussel, 92; clam, 106. 
Sessile: fixed to one place, without locomotory powers — of an animal; 
Bugula, 85; oyster, 99; Molgula, 135 ; tubularian, 163 ; campanularian, 
169; Grantia, 181; Vorticella, 188: not on a stalk or stem — of an 
organ, 46. 
Seta: a bristle; Nereis, 61; earthworm, 67. 
Setigerous glands: glands which secrete sete, 73. 
Sexual: reproduction through the agency of the two sexes. 
Shell gland: the kidney of entomostracans; copepod, 55; Daphnia, 57. 
Siphon: the organ through which water enters or leaves the mantle cavity 
in mollusks and ascidians; mussel, 92; clam, 106; squid, 124; Mol- 
gula, 135. 
Siphonoglyph : a ciliated groove in the angle of the gullet in Anthozoa, 178. 
Somite: one of the serial, homologous body-segments which form the 
body of an articulate animal. 
Spermatheca : a sac for the storing of sperm in the female animal, a semi- 
nal receptacle, 72. 
Spermatophore: a capsule or mass of spermatozoa; copepod, 55; snail, 121; 
squid, 133. 
Spermatozoon: the male sexual cell. 
Sperm-duct : the vas deferens, 27, 71. 
Sperm-sac: a sac for the storing of sperm in the male animal, a seminal 
vesicle, 71. 
Sperm-sphere : a mass of spermatozoa in the earthworm, 73. 
Spicule : a minute calcareous or silicious body in sponges and echinoderms. 
Spinnerets: the appendages on a spider from which the silk exudes, 25. 
Spiracle: an external opening in the tracheal system; wasp, 3; fly, 8; 
grasshopper, 11; caterpillar, 21; spider, 26. 
Sporosac: a sessile medusoid, one which remains attached to the parent 
hydroid; tubularian, 163; campanularian, 169. 
Sternite: the ventral portion of an abdominal segment in insects; wasp, 3; 
grasshopper, 10. 
