Glossary 



389 



and vertebrates. 



TUNIC. In botany and zoology, 

 any well differentiated membran- 

 ous covering of an organ or an 

 entire organism; much the same 

 as a test. 



TUNICATE. Name of a group of 

 marine animals, most sharply 

 characterized by the cellulose- 

 containing tunic, or test, which 

 envelopes the body; by the pe- 

 culiar basket-like respiratory sys- 

 tem; and by the notochord or 

 precursor of the vertebral col- 

 umn, possessed by all the species 

 in the embryonal life, and by a 

 few during the whole life, fre- 

 quently used synonymously with 

 Ascidian, which see. 



VAGAI,. Pertaining to the vagus 

 nerves, one of the tenth pair of 

 cranial nerves in all true verte- 

 brates. 



VASO-CONSTRTCTOR. Applied to the 

 nerves which cause contraction 

 of the walls of blood vessels. 



VASO-DILATOH. Applied to nerves 

 which cause, or more exactly, 

 permit a widening of the blood 



vessels by diminishing the tonus 

 of the muscles of the vessel 

 walls. Since the smaller blood 

 vessels are all supplied with both 

 constrictor and dilator nerves 

 the constant balancing between 

 these antagonistic influences, 

 both kinds of impulse being in 

 response to the general needs of 

 the organism, this scheme illus- 

 trates well a principle of equili- 

 bration widely operative in the 

 animal kingdom. 



VISCERAL. A term used in zoology 

 to indicate not only the totality 

 of internal organs, but also the 

 side of .the animal on which 

 these are situated. 



ZYGOTE. A body formed by the 

 conjugation of two reproductive 

 cells, called gametes. Gametes 

 and zygotes may be either uni- 

 cellular organisms, or the repro- 

 ductive cells of multicellular or- 

 ganisms. 



ZYMOGEN. The enzyme-producing 

 substance in the secretory cells 

 of glands the secretions of which 

 contain enzymes. 



