372 PRINCIPLES OF ANIMAL BIOLOGY 



Avoiding reaction. The behavior by which Paramecium avoids obstacles of various 

 kinds. It consists of stopping, moving backward, turning through an angle away 

 from the oral groove, and starting forward in a new direction. 



Axial skeleton. The skull, vertebral column, ribs, sternum, and hyoid apparatus of 

 vertebrates. 



Axolotl {aks' lot'I). The larval form of the tiger salamander Ambysioma tigrinum 

 which reproduces while in the larval state. 



Axon {aks' one). A projection from a nerve cell which ordinarily conducts impulses 

 away from the body of the cell. 



Backcross. A cross between an Fi individual and one of its parent types. 

 Balanoglossus. A genus of wormlike animals doubtfully included in the phylum 



Chordata. 

 Bascanion. A genus of snakes, including the black snake or blue racer. 

 Base. A substance giving rise to free hydroxyl ions, 0H~, and thereby accepting 



hydrogen ions, H"*". 

 B complex. A group of related vitamins found in meats, seed coats of cereals, yeast, 



etc., including thiamin, riboflavin, niacin, and pyridoxin. 

 Biconcave. Having the centrum hollow both in front and behind; said of vertebrae. 

 Bidder's canal. A longitudinal tube near the median border of the kidney of certain 



Amphibia; into it the collecting tubules open. 

 Bilateral symmetry. An arrangement of the parts of an object or animal body such 



that the halves on opposite sides of a certain plane (only one in number) are 



mirrored images of each other. 

 Bile. The fluid secreted by the liver in vertebrates. 



Bile duct. The tube through which bile is di.scharged into the intestine. 

 Binomial. Consisting of two names or terms. Applied to the system of nomencla- 

 ture by which each species is given two names, one for the genus, the other for 



the species. 

 Biogenetic law. The doctrine that animals in their embryonic development repeat 



the evolutionary history of the race. 

 Biology. The science of life and of living things, whether plants or animals. 

 Bladder. A membranous sac in which urine is stored. 



Blainville, Henri Marie Ducrotay de (5/aN veel'). French naturalist, 1777-1850. 

 Blastocoele (bias' to seel). The hollow interior of a blastula. 

 Blastopore. The opening through which the archenteron of an early embryo (gas- 



trula) communicates with the exterior. 

 Blastostyle. In hydroids, a nontentaculate individual which produces medusae. 

 Blastula {bias' tu La). An early developmental stage, consisting of a hollow ball of 



cells. 

 Blood platelet. One of the formed components of the blood, produced by fragmenta- 

 tion of certain cells. 

 Book gill. See book lung. 

 Book lung. A respiratory organ composed of flat sheets joined together like pages of 



a book, found in spiders. 

 Bougainvillea ramosa {boo' gin viV le a). A species of marine hydroid. 

 Bowman's capsule. The expanded end of a kidney tubule, in which a glomerulus is 



located. 

 Brachiopoda {brak' i op' o da). A group of marine animals of uncertain rank or 



relationship. They have a bivalve shell, the two halves of which are unequal. 



Sometimes placed in a phylum with the Bryozoa and Phoronidea. 

 Bract. One of the covering (protective?) members of a siphonophore colony. 



