266 GLOSSARY 



Stomodaeum. The anterior portion of the alimentary canal which is 



ectodermal in origin. 

 Stone canal. The tube which leads from the madreporic plate to the 



circular water canal in echinoderms. 

 Stylet. A small, sharp instrument. 

 Subgenital Pits. The pouches adjacent to the gonads of the Dis- 



comedusae, on the subumbrellar side. 

 Subumbrella. The concave or oral side of a medusa. 

 Sulcus. A furrow or groove. 



Suture. An immovable union between plates or ossicles. 

 Swimmeret. Pleopod; any abdominal appendage of a crustacean. 

 Swimming plate. One of the ciliated swimming organs of a ctenophore. 

 Synchronous. Happening at the same time. 



Tactile. Capable of feeling. 



Tarsus. The segmented foot of an insect. 



Telson. Hinder division of a crustacean, usually not considered a seg- 

 ment. 



Tentacle. An elongated, tactile organ, usually unsegmented. In poly- 

 chaetes, a prostomial structure usually arising from the anterior 

 end and more dorsal than the palps. 



Tentaculocyst. A marginal sense organ in certain medusae. 



Tergum. The dorsal covering of a segment of an arthropod. 



Test. Hard or firm outer covering as the shells of many protozoa, 

 echinoderms, molluscs, etc., and the tunics of Urochorda. 



Testis. Sperm-producing organ. 



Thorax. The body division of arthropods between the head and ab- 

 domen. 



Tibia. In an insect leg, the segment lying between the femur and the 

 tarsus. 



Trachea. One of the respiratory tubes of certain arthropods. 



Trichocyst. An ectoplasmic organelle, often considered as offensive or 

 defensive in function, formed in many holotrichous ciliates. 



Trivium. The three rays of a starfish which are farthest from the 

 madreporic plate. 



Trochal disk. The ciliated disk of a rotifer. 



Trochanter. In an insect leg, the second segment from the body of 

 the animal. 



Trochophore. The free-swimming larva, with bands of cilia, especially 

 typical of the annelids and Mollusca. 



Trochosphere. Trochophore. 



Tubercle. A small knoblike prominence. 



Tunic. The outer covering of an ascidian. 



