GLOSSARY 



Abdomen. The most posterior body- division in arthropods and in some other 



invertebrates. 



Aboral. The side of the body opposite the mouth in a radiate animal. 

 Aciculum. A chitinous supporting rod in the parapodia of annelids. 

 Aoontium. A vibratile thread-like organ charged with nettle cells in certain 



sea anemones. 



Acraspedote medusa. A medusa without a velum. A scyphomedusan. 

 Acrocyst. A brood-chamber in certain campanularian hydroids. 

 Actinule. A larval form of certain hydroids. 

 Adaptive gills. Projections on certain opisthobranchs which function as gills, 



but are not ctenidia. 

 Adductor muscle. A muscle which draws an organ towards the axis of the 



body. 



Algae. Very simple green plants. 

 Alimentary tract. The digestive canal, the organ which ingests, digests, and 



absorbs the food. 

 Alternation of generations. The alternate succession of sexual and asexual 



generations in a species of animals or plants. 

 Ambulacral feet. Tubular projections of the body wall with sucker discs at 



their ends in echinoderms. 

 Ambulacral groove. The elongated groove on the oral side of the rays of the 



starfish. 

 Ambulacral pores. Minute openings in the body-wall in the starfish and the 



sea-urchin. 



Ampulla. A sac-like projection of the ambulacral foot in echinoderms. 

 Anal feelers. Paired posterior appendages which are sensory in function. 

 Analogous. Having a similar function. 

 Antenna. A segmented sensory appendage on the head. 



Antennal scale. The exopodite of the second antenna in certain malacostracans. 

 Antennal sinus. An indentation in the shell near the antennae of certain 



ostracods. 



Anterior* At or towards the front end of the body. 

 Anus. The posterior opening of the digestive canal. 

 Aorta. A large artery leading directly from the heart. 

 Aperture. The opening of a snail shell. 

 Apex. The tip of a snail shell. 



Appendage. A projection from some part of the body. 

 Appendix. A short diverticulum usually of the intestine. 

 Archeocytes* Primitive cells in the mesoglea of sponges. 

 Aristotle's lantern. The dentary apparatus of the sea-urchin. 

 Artery. A blood vessel carrying blood away from the heart to the tissues. 

 Arthrobranch. A gill attached to the joint between the leg and the body in 



crustaceans. 



Articulate. Composed of a series of homologous segments. 

 Asexual. Reproduction by division or budding and not through the agency 



of the sexes. 



AtoTce. The anterior, sexless part of certain annelids. 

 Auricle. A chamber of the heart which receives the blood from the veins. 



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