GLOSSARY , 371 



Appendicular skeleton. The bones of the Hmbs and their attaching girdles in 



vertebrates. 

 Arachnida (a rak' ni da). A class of Arthropoda comprising the spiders, scorpions, 



and mites. For definition see Chap. 19. 

 Archaeozoic {ar' he o zo' ik). Of the earliest geological era; the oldest known rocks 



are of this era. 

 Archenteron {ark en' ter on). The cavity within the endoderm of a gastrula. It 



communicates with the exterior. 

 Archiannelida {ar' ki an nel' i da). A class of primitive marine worms (Annelida) 



without setae. 

 Aristotle {ar' is tot' I). The most famous of the Greek naturalist philosophers, who 



lived 384-322 b.c. 

 Armadillo. An armored mammal of the order Edentata, which includes also the 



sloths and anteaters. 

 Arteriole. One of the smaller branches of an artery, leading to capillaries. 

 Artery. A blood vessel conducting blood from the heart. 

 Arthropoda {ar throp' o da). A phylum of animals, including the insects, Crustacea, 



centipedes, etc. For definition see Chap. 19. 

 Articulate. To join; said of bones. 



Artiomorphes {ar' ti o mor' feez). A group of animals in Blainville's early classifica- 

 tion; it comprised the animals whose bodies are bilaterally symmetrical. 

 Ascaris (as' ka ris). A genvis of roundworms (Nemathelminthes) parasitic in various 



animals. A. megalocephala {meg' a lo scf a la), parasitic in the intestine of the 



horse. 

 Ascorbic acid. Vitamin C, the preventive of scurvy. 

 Asexual. Not involving germ cells or fusion of nuclei; said of reproduction, or of an 



individual employing such a mode of reproduction. 

 Assimilation. The conversion of digested foods and other raw materials into proto- 

 plasmic substances. 

 Association neuron. A nerve cell within the central nervous system, which helps to 



connect an afferent with an efferent neuron. 

 Aster. The starlike figure composed of a centriole and the radiating lines about it; 



or the centriole may be lacking. 

 Asteroidea {as' ie roi' de a). A class of Echinodermata comprising the starfishes. 



For definition see Chap. 19. 

 Astral rays. The radiating lines surrounding a centriole in a dividing cell. 

 Asymmetry. Absence of any kind of symmetry. 

 Atoll {at' ol, or a toV). A ring- or horseshoe-shaped coral island. 

 Atom. A unit of a chemical element, composed of one or more protons and electrons, 



arid usually neutrons. 

 Auditory. Pertaining to hearing; applied to the nerve of hearing and the sensory part 



of the inner ear. 

 Auricle. The anterior chamber of the heart in fishes, and one of the two anterior 



chambers in higher vertebrates. 

 Autonomic nervous system. A system of ganglia and nerve fibers, comprising two 



mutually antagonistic groups, which center in specific parts of the central nervous 



system and regulate the involuntary responses of the heart, blood vessels, diges- 

 tive tract, glands, and pupil of the eye. 

 Autosome {aw' to some). Any chromosome not closely associated with sex, that is, 



not an X or Y chromosome. 

 Aves {a'veez). A class of vertebrate animals comprising the birds. 



