GLOSSARY 493 



Pineal system (L. pinus, a fir cone) : the term applied to the parietal sense- 

 organ (or organs) and the associated structures, such as the parapineal 

 organ, pineal sac, the pineal stalk, the epiphysis, the pineal nerves, 

 commissures, and habenular ganglia. 



Planaria (L. planus, flat) : a small flat worm, showing bilateral symmetry and 

 having paired ocelli. Phylum Plathelminthes. 



Podophthalmata (Gr. pons, foot ; ophthalmos, eye) : a division of the Crus- 

 tacea having compound eyes borne at the free ends of movable stalks. 

 It comprises two Orders, Stomatopoda (e.g. Squilla) and Decapoda 

 (e.g. Astacus). 



Polyclada (Gr. polus, many ; dados, branch) : an order of the flat worms 

 belonging to the class Turbellaria, so named on account of the complex 

 branching of the intestine. 



Polyphemus (Gr. the cyclopean giant described by Homer) : a small crustacean 

 belonging to the order Cladocera, in which the eyes are sessile and are 

 joined into a single organ. 



Porthetis spinosa (Gr. porthetes, destroyer ; L. spinosa, thorny) : an insect 

 belonging to the group Acridiidae, in which there is a coexistence of the 

 compound and simple eyes in the imago. The three frontal ocelli are 

 arranged in a triangle, one in front, two behind. 



Pterichthys (Pterychthys) (Gr. pteron (pterux), wing ; ichthus, fish) : an 

 extinct fish belonging to the order Antiarchi of the class Ostracodermi. 

 It is characterized by wing-like pectoral fins. 



Pterygotus (Gr. pterux, wing ; ous, ear) : a genus of the Eurypterida belonging 

 to the order Merostomata. The antennae ended in prehensile lobster- 

 like claws ; the maxillipedes formed large ear or oar-like appendages 

 which could be used as paddles. 



Retinula (L. diminutive of rete, net) : the inner sensitive segments of the 

 ommatidia of an aggregate or compound eye, as contrasted with the 

 outer, purely dioptric part. Some writers include the pigment cells, 

 which surround the clear cells of the rhabdome, whereas others appear 

 to limit the term to the clear central cells of the ommatidium. 



Rhabdite (Gr. rhabdion, a little rod) : the central columnar cell of a retinula. 

 The basal part of the cell containing the nucleus is usually expanded, 

 whereas the distal part tapers or is prolonged as a slender refractile 

 rod. 



Rhabdome (Gr. rhabdos, rod) : the central clear cells of a retinula, including 

 the rhabdite and the peripheral non-pigmented cells. 



Rhabdomere (Gr. rhabdos, wand or rod ; meros, part) : one of the constituent 

 cells or rods of a rhabdome. 



Rhipidoglossa (Gr. rhipis, a fan, something thrown out, as a javelin ; glossa, 

 tongue) : the name given to the chameleon family on account of the 

 rapid movement of the long tongue, which is used for catching flies. 



Rhynchonella (Gr. diminutive of rhunchos, a small beak) : an articulate 

 brachiopod in which the dorsal and ventral valves of the shell are joined 

 by a horizontal hinge with a beaked process. Found in lower cretaceous 

 strata, and closely resembles the shells of certain living species. 



