EXPLANATION OF THE LETTERING. 479 



op.tr.h Optic tract to hemispheres. 



o.r Lateral eye orbit. 



os Cardiac ostia. 



o.sh Outer sheath. 



ov Ovaries. 



p External opening of parietal eye tube; epiphysis; anterior 



neuropore; opening to endolymphatic ducts; penis 



thoracic appendages. 



pa. Parietals. 



p.ap. Pectoral appendage. 



pa.ey Parietal eye. 



pa.ey.g Parietal eye ganglion. 



pa.ey.n 1 " 2 Roots to parietal eye nerves. 



p.b Posterior branchial line of canal organs. 



p.b.c Peribranchial chamber. 



p.c Paccinian-like corpuscles. 



p.c Pore canals; proboscis ccelom. 



pc.a Pectoral arch. 



p.ca Pulp cavity of denticle. 



p. card Posterior cardinals. 



p.ce .Prosencephalon. 



p.d . Pericardium. 



p.dl. Posterior dorsolateral plate. 



p.d.p Posterior mandibular dental plate. 



p.e Parietal eye. 



p.e.n. Parietal eye nerve. 



pec Pectines. 



ped.g Pedal ganglion. 



p.e.p Pits on inner surface of postorbital plate; posterior parietal 



eye pits. 



per.c Pericardial nerve. 



p.e.t Parietal eye tubercle. 



p.g. ... .Polar globules. 



ph. . Pharynx. 



p.h.co . Posthaemal commissures. 



pit.f .Pituitary foramen; passage-way for primitive stomodaeum. 



pl.ap. ...... Pelvic appendage. 



pi. ex Plastro-coxal muscles. 



pl.ent. . Plastro-entapophyseal muscle. 



pl.f. .Pleural fold. 



pl.t Plastro-tergal muscle. 



p.m. . . ..... .Posterior marginal. 



p.m.v Posterior median-ventral plate. 



p.n .Pedal nerve. 



p.n.co .Posterior neural commissure. 



p.np Posterior neuropore. 



p.o.c. Preoral chamber. 



p.p .Posterior process; parietal eye plate; proboscis pore. 



pr Prepuce; cephalic mantle. 



pr.mx. or p.mx Premaxillae. 



pr.n or pro Pronephros. 



