GLOSSARY. 221 



Ocellus. A simple eye of an arthropod. 



Ocular plate. A plate at the end of an ambulacral area of an echino- 



derm. 

 Olfactory organ. An organ to distinguish odors. 

 Ooecium. A structure in Polyzoa in which the pmbryo develops. 

 05type. The space in flat-worms where the eggs are supplied with shells. 

 Operculum. The horny lid that fits into the aperture of the shell of some 



gastropods. 

 Oral. The mouth side. 



Osculum. The opening of a sponge through which water escapes. 

 Osphradium. A supposed sense organ of Mollusca. 

 Ossicle. A small hard plate. 

 Ostium. A small pore; in lamellibranchs one of the pores in the gills 



through which water is passed. 

 Otocyst. A sense organ, probably static in function. 

 Otolith. A hard body in an otocyst. 

 Ovary. A female sexual gland. 



Oviducal gland. A glandular division of an oviduct. See squid. 

 Oviduct. A female sexual duct that leads from the ovary. 

 Ovipositor. A modified portion of some insects that is used in depositing 



eggs. 

 Ovum. Female germ cell. 



Pallial line. The depression in the shell of a lamellibranch that is caused 

 by the attachment of pallial muscles. 



Pallial sinus. The indentation in the pallial line of some lamellibranchs, 

 caused by the insertion of the retractor muscles of the siphons. 



Pancreas. A digestive gland. 



Papilla. A small projection. 



Paragnatha. Lamellae behind the mandibles of some Crustacea. 



Parapodium. An appendage on a somite of an annelid. 



Parenchyma. A soft tissue; that which fills the body-cavities of flat- 

 worms. 



Pectine. One of a pair of appendages of scorpions. 



Pedicellaria. A minute pincer-like organ that is present on some 

 echinoderms. 



Pedipalpi. Appendages of Arachnoidea. 



Peduncle. A short stalk. 



Pelagic. Organisms that live at or near the surface of the water. 



Pen. Vestigial shell of a cephalopod. See squid. 



Penis. Male intromittent organ. 



Pericardium. A membrane surrounding the heart. 



Periopod. A walking-leg of a crustacean. 



