264 GLOSSARY 



Respiratory tree. The respiratory mechanism of some holothurians, 



opening into the cloaca. 

 Retractor muscle. A muscle that withdraws an organ or part of an 



animal. 

 Root stalk. A creeping stem from which the hydrocauli originate. 

 Rostrum. The anterior spine of a lobster and of other crustaceans. 

 Rotula. One of the calcareous pieces of the jaw of an echinoid. 

 Rudimentary. When applied to adult animals, means permanently 



undeveloped; vestigial. 



Sagittal plane. The median plane, i. e., that which divides a bi- 

 laterally symmetrical animal into two equivalent halves. 



Salivary gland. In invertebrates, any gland which opens into the 

 mouth cavity. 



Scaphognathite. The flattened structure composed of the fused endo- 

 pod and epipod of the second maxilla of certain Crustacea. It is 

 used to expel water from the branchial cavity. 



Schizogony. Spore formation of the type characteristic of schizonts. 



Schizonts. In Sporozoa a cell, formed by the growth of a sporozoite 

 (merozoite) in a cell or corpuscle of the host, which forms mero- 

 zoites by sporulation (multiple fission). 



Schizopod. A biramous arthropod appendage. 



Scolex. Anterior or attaching portion of the tapeworm from the pos- 

 terior end of which the proglottids are formed. 



Scyphistoma. The attached, hydra-like larval stage of many Scy- 

 phozoa. 



Segment. One of a series of divisions of an animal's body or appendage. 

 In the former case it is synonj^mous with somite. 



Segmentation, (a) Cleavage; (6) division of body into segments. 



Seminal receptacle. A sac in which spermatozoa are stored. 



Seminal vesicles. The sacs which inclose the testes of an earthworm. 



Septum. A plate which divides two spaces. In annelids the partition 

 which separates the coelomic cavities of adjacent segments. 



Sessile. Attached; without the power of locomotion. Also applied to 

 hydranths without a stalk. 



Seta. A stiff, slender, bristle-like structure. Most commonly applied 

 to one of the chitinous bristles imbedded in the body wall or 

 parapodium of an annelid. 



Setigerous gland. An integumentary gland which forms setae. Typi- 

 cally two pairs per segment. 



Shell gland. A gland which secretes the egg shell; sometimes applied 

 to the excretory organs of Entomostraca. 



