GLOSSARY 



This glossary is not a complete dictionary of zoological terms or even of all such terms used 

 in this book. Words that are used in a limited part of the book are defined where they occur, 

 and the student is referred to the Index in such cases. 



A-. Combining form, meaning "witliout." 



ABDOMEN. That part of the trunk of a vertebrate which is posterior to the thorax 



and separated from it by the diaphragm, and which contains that part of the 



coelom known as the abdominal or peritoneal cavity. In the arthropods, the 



posterior division of the body. 

 ABORAL. Opposite the mouth. 



ABSORPTION. The taking in of soluble foods by the circulating fluids. 

 ACOELOMATE. Animals that are bilateially symmetrical and triploblastic but have 



no body cavity of any kind between the gut wall and the body wall. 

 ACQUIRED CHARACTER. A phrase used as a synonym for somatic variation or fluctu- 

 ation in Lamarck's Theory of the Inheritance of Acquired Characters. Since 



all the characters of any individual are acquired as the zygote develops, the 



phrase is an unfortunate one. 

 ACTIVATION. The initiation of development, usually the result of the penetration of 



an ovum by a spermatozoon. 

 ADAPTATION. Any modification of an organism that better fits it for existence in its 



present environment or enables it to live in a different environment. 

 ADDUCTOR MUSCLE. A muscle that draws a part toward a central region or median 



line. 

 ADSORPTION. The accumulation of particles on a surface. 

 AFFERENT. Carrying to or toward a given region. Cf. EFFERENT. 



AGAMETE. Any single cell, other than a gamete, that can reproduce the organism. 

 AIR TUBES. Tubes conveying air to the lungs in vertebrates and from the spiracles 



to the cells in certain arthropods. 

 ALBUMEN. The white of the reptile's and bird's egg, surrounding the yolk, or zygote, 



and used as food by the embryo. 

 ALGAE (sing. ALGA). A group of green plants with relatively simple organization. 

 ALLELOMORPHS. Genes located in comparable positions on homologous gene strings 



but producing alternative effects in a given character; for example, gene A might 



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