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^"^ 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 coelom. 



pea 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 Phar3mx. 



p.h.co Postha?mal commissures. 



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



pl.ap Pelvic appendage. 



pl.cx 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. orp.mx Premaxillae. 



pr.n or pro Pronephros. 



