460 ELEMENTS OF BIOLOGY 



Spherical symmetry. The structure of the organism is such that all 

 parts are symmetrically placed with reference to a point. 



Spiracle, (a) An aperture in the body wall of insects and some spiders 

 that leads into the respiratory tubules. 

 (b) A modified anterior visceral cleft in the elasmobranch fishes. 



Spiral valve. A winding shelf within the small intestine of elasmo- 

 branch fishes, for example, the shark. 



Splanchnic mesoderm. The inner sheet of mesoderm that in the 

 embryo adheres closely to the endoderm and forms the inner wall 

 of the coelom. 



Spongin. An organic substance which forms the chief ingredient in the 

 supporting framework of some sponges, for example, the common 

 bath sponge. 



Sporangium. The structure on the spore-bearing generation of plants in 

 which the spores are produced. 



Spore. A single cell capable of developing into an adult without being 

 fertilized. Among animals only the Protozoa form true spores. 

 Spores are usually enclosed in a protecting case; when at rest their 

 life processes go on very slowly. 



Sporophyte. The stage in the life cycle of mosses, ferns, and seed-bear- 

 ing plants which produces spores and reproduces only a sexual 

 generation solely by the development of spores. 



Stamen. A highly modified leaf that in the flower bears the organs 

 which produce microspores. 



Stereotropism. Response to contact. 



Stimulus. Any agent or condition that causes living matter to respond. 



Stratum germinativum. A layer of cells forming the inner and grow- 

 ing portion of the epidermis. 



Structural adaptations. Anatomical characters that are adaptive in 

 nature. 



SuDiFERous GLANDS. The sweat glands. 



Symbiosis. The term in its original meaning broadly includes the living 

 together of two organisms. Its meaning tends to be confined to 

 those cases in which the two organisms are intimately associated 

 and do not harm each other. 



Synapse. The structure that forms the contact between axone and 

 dendrite. 



Synapsis. Refers to the behavior of members of chromosome pairs dur- 

 ing their period of contact in the maturation of the germ cells. In 



