Glossary 



S89 



and vertebrates. 



Tunic. In botany and zoolo/ry, 

 any well differentiated membran- 

 ous covering of an organ or an 

 entire organism ; mueb tbe 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. 



Vagal. 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-dtlator. Applied to nerves 

 which cause, or more exactly, 

 permit a widening of the blood 



vessels by (liiniiiisiiing the tonus 

 of the musclrs of tin- vrssel 

 walls. Since the smaller blood 

 vessels are all supplied with iKith 

 constrictor and dihitor nrrvrs 

 the constant balancing between 

 these nntngonistic influences, 

 l)oth kinds of im])ulse In-ing in 

 rcsjionse to the general needs r>f 

 the organism, this sclK-me illus- 

 trates well .'i principle of equili- 

 bration widely operative in the 

 animal kingdoni. 



ViscKRAi.. 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. 



Zvm(k;kn. The en/.yme-producing 

 substance in the .secretory cells 

 of glands the secretions of which 

 contain enzvmes. 



