252 GLOSSARY 



Ambulacral pore. The opening through which a tube foot projects. 



Ambulacral ridge. The elevation in the coleom of starfish arm, caused 

 by the ambulacral plates. 



Ambulacral sucker. The sucker at the end of a tube foot. 



Amphiblastula. A characteristic embryonic stage of a sponge. 



Ampulla. A reservoir connected with the tube foot of an echinoderm. 



Anal plate. In the periproct of an echinoid, the plate in which the 

 anus lies. 



Analogous. Similar in function. 



Anastomosis. In the simplest form, a cross connection between two 

 adjacent blood vessels or nerves. Frequently many such cross con- 

 nections among several related vessels or nerves result in the forma- 

 tion of a network, 



Annulation. A ringlike part or annulus as in the ringed stem of cer- 

 tain hydroids. 



Annulus. A ring or ringlike part or structure. 



Antenna. A sensory head appendage of an arthropod. 



Antennule. A sensory paired head appendage of an arthropod, placed 

 just anterior to the antenna when present. Usually designated as 

 first antenna. 



Anterior. Front or head end. 



Anteroposterior. Lengthwise of the body. 



Anus. The opening (usually posterior) of the alimentary canal through 

 which the feces are discharged. 



Apical plate. An ectodermal thickening at the anterior end of trocho- 

 phore larva. 



Apical system. A group of plates surrounding the periproct of an 

 echinoid. 



Apical tuft. A group of large cilia on the apical plate. 



Apopyle. The opening of a radial canal of a sponge into the gastric 

 cavity or cloaca. 



Arthrobranch. A gill of a crustacean, borne by the articular membrane 

 at the base of an appendage. A joint gill. 



Asexual reproduction. Keproduction without sexual phenomena, e. g., 

 binary fission, multiple fission (sporulation), budding, etc. 



Atriopore. External opening of the atrium. 



Atrium. In tunicates and Amphioxus, a chamber partly enclosing the 

 pharynx and receiving' water from the latter through the pharyngeal 

 slits. Genital products and, in adult tunicates, feces also pass into 

 the atrium. 

 Auricle. A receiving chamber of the heart. 



Avicularium. A structure shaped like a bird's head, present in some 

 Bryozoa. 



