194 GLOSSARY. 



Anterior. Front or head end. 



Antero-posterior. Lengthwise of the body. 



Anus. The posterior opening of the alimentary canal. 



Aorta. In invertebrates used to designate the chief blood-vessel. 



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



echinoid. 

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



cavity or cloaca. 

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



at the base of an appendage. 

 Asexual. Reproduction by other than sexual methods, as by budding 



or division. 



Atriopore. External opening of the atrium of Amphioxus. 

 Auricle. A division of the heart. 

 Avicularium. A structure shaped like a bird's head, to be found on 



some of the Polyzoa. 

 Axial organ. A structure near the stone canal of echinoderms that is 



apparently connected with the genital organs. 



Basipod. Second segment from the body of a crustacean limb. 



Beak. Horny mouth parts; the point from which growth has proceeded 

 in a clam shell. 



Bilateral symmetry. Right and left sides alike. 



Biramous. Composed of two branches, as a typical crustacean appendage. 



Bivalve. Having two valves or pieces, as a clam shell. 



Bivium. The two rays of a starfish that are nearest the madreporic 

 plate. 



Blastostyle. The axial tissue of a reproductive polyp of certain Hydrozoa. 



Body-cavity. Crelom; the cavity between the alimentary canal and body- 

 wall. 

 Body-wall. The outer wall of the body. 



Brain. In invertebrates frequently applied to the cerebral ganglia. 



Branchiae. Gills; organs adapted for aquatic respiration. 



Branchial heart. An accessory heart placed at the base of the gill, as in 

 the squid. 



Brood sac. A cavity or pouch in which developing embryos are carried. 



Bud. An outgrowth of an animal that will become a new individual. 



Byssal thread. One of the threads by which certain lamellibranchs attach 

 themselves. 



Caecum. A sac-like appendage of the alimentary canal. 

 Calciferous glands. Esophageal glands of some annelids. 

 Carapace. Head and thoracic covering of some crustaceans. 



