492 THE PINEAL ORGAN 



Parapineal organ (Gr. para, beside ; L. pinus, fir cone) : the anterior or left 

 parietal sense-organ of Petromyzon. It lies beneath the right parietal 

 sense-organ and is smaller and less differentiated than the right organ. 



Parietal (L. paries, wall) : 



bone: roof-bone of skull. 



eye : visual sense-organ. 



" fleck " : white spot visible through cornea. 



foramen : complete parietal canal. 



pit : incomplete parietal canal. 



plate : separate bone in pineal region. 



" plug " : translucent tissue closing up pineal canal. 



region : pineal region of roof of third ventricle. 



scale : horny scale overlying pineal region of skull. 



sense-organ : terminal sensory-vesicle. 



vesicle : dilated distal end of pineal stalk. 



Patella (L. Patella, a small pan or dish) : the limpet, a gasteropod mollusc 

 having a low conical shell. The paired eyes of the adult animal are open 

 pits, without a lens. The larva is a typical free-swimming trochophore. 



Peripatus (Gr. peripateo, walk about) : an aberrant arthropod resembling a 

 caterpillar ; neither body nor legs are definitely segmented. The eyes 

 are of a simple ocellar type, and resemble those of Nereis. 



Permian System or Period : the uppermost strata of the Palaeozoic or Primary 

 Era — found in the district of Perm, East Russia. It consists of sand- 

 stone, marls, rocky salt, and magnesium limestone ; the conditions 

 were chiefly continental and desert. 



Phascolosoma (Gr. phaskolos, leather bag or purse ; soma, body) : a genus of 

 the suborder Sipunculidae, class Annulosa. Eyes are absent in the 

 adult animal ; a pair of ocelli are, however, present in the trochophore 

 larva. 



Phyllopoda (Gr. phullon, leaf ; pous, foot) : an order of the Crustacea having 

 leaf-like legs and comprising Apus, Lepidurus, and the fairy-shrimp or 

 Branchipus diaphanus. The order resembles in many respects the 

 extinct Trilobita. 



Phylogeny (Gr. phulon, race ; genesis, origin) : the ancestral origin of a phylum 

 or subdivision of a phylum. 



Pineal body (L. pinus, a fir cone) : the epiphysis cerebri, so named on account 

 of its conical shape in man and certain animals. 



Pineal foramen : the canal which contains the pineal organ ; it is usually 

 situated between the parietal bones or in the centre of a single bone 

 formed by fusion of the parietal bones. It may, however, be situated 

 between the two frontals, in a single frontal bone or in a special pineal 

 plate. 



Pineal sac : the dilated distal end of the " epiphysis," sometimes termed the 

 end-vesicle. It is well developed in Sphenodon, contains pigment, and 

 it was believed by Dendy to represent the right member of a pair of 

 parietal sense-organs of which the left, or pineal eye, is more highly 

 developed and lies in the parietal foramen. 



