GLOSSARY. 



Abdomen. The posterior division of the body of an arthropod. 



Aboral. The surface away from the mouth. 



Aciculum. A supporting rod in a parapodium of an annelid. 



Acinous. Saccular or granular. 



Acontium. One of the filaments that are attached to the mesenteries 



of such forms as Metridium. 

 Actinula. A larval stage of a hydroid. 

 Adductor muscle. A closing or withdrawing muscle. 

 Adhesive organ. A sucker or sticky pad that will adhere. 

 Ad-radial canal. A canal in a medusa that lies between adjacent per- 



and inter-radial canals. 



Afferent. Coming toward, as a vessel that leads to an organ. 

 Alga. A simple plant. 

 Alimentary canal. Digestive tube. 



Alternation of generation. Alternation of sexual and asexual genera- 

 tions in the life cycle of an organism. 

 Alveolus. A little sac or cavity; also one of the plates that bears the 



teeth in an echinoid. 



Ambulacral area. The region bearing the tube feet of an echinoderm. 

 Ambulacral foot. A tube foot of an echinoderm. 

 Ambulacral groove. One of the depressions in which the tube feet of 



a starfish are placed. 



Ambulacral plate. One of the plates of an ambulacral area. 

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

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



by the ambulacral plates. 



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

 Amphiblastula. An embryonic stage of a sponge. 

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

 Anal plate. One of the plates in the periproct of an echinoid. 

 Analogous. Similar in function. 



Anastomosis. The joining together, as of vessels and nerves. 

 Antenna. A sensory head appendage of an arthropod. 

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



anterior to the antenna when present. 



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