GLOSSARY 



Note: The definitions given below are, of course, intended pri- 

 marily to apply to the invertebrates. With a few exceptions no attempt 

 has been made to supply the meanings which apply to the vertebrates. 



Abdomen. In invertebrates, the posterior division of the body. 

 Aboral surface. The surface of the body opposite the oral or mouth 



surface. 

 Aciculum. A supporting rod in an annelid parapodium. 

 Acinous. Saccular or granular. 



Acontium. In sea anemones, a threadlike organ containing nettle cells. 

 Acraspedote medusa. A medusa without velum. Typical of Scyphozoa. 

 Acrocyst. An extracapsular brood chamber attached to distal end of 



gonosome in certain calyptoblastic hydroids. 

 Actinopharynx. Tube leading from mouth to coelenteron in sea 



anemones. 

 Actinule or actinula. A specialized larval form, having aboral and oral 



tentacles and developed in the medusa of tubularian hydroids. 



Ultimately gives rise to a new colony. 

 Adductor muscle. A closing or withdrawing muscle. 

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

 Adnate. Said of hydranths growing with one side adherent to a stem. 

 Adradial canal. In a medusa, a canal lying between adjacent per- and 



inter-radial canals. 

 Afferent. Carrying toward, as a vessel which leads to an organ. 

 Alga. A simple cholorphyl-bearing plant. 

 Alimentary canal. Digestive tube. 



Alternation of generations. 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. 



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