478 EXPLANATION OF THE LETTERING. 



m.n.co Median neural commissure. 



m.oc Median ocellus. 



m p JVJalpighian tubules; madreporite. 



m .pl Medullary plate. 



m.r Marginal wall or ridge. 



ms. Mesoderm. 



msc Mesenchyme. 



ms.en Mesentoderm. 



m.s.p.ent Mesoplastro-entapophyseal muscle. 



m.t Motor nerve tubes. 



mt Mantle. 



mx Maxillae. 



mx.d Excretory duct to maxillary segment. 



mxl Maxillaria. 



n. or n.c Nerve cord; neuron; neuromere. 



n.ch.c Neurochordal canal. 



n.co Neural commissure. 



n.cr Neural crests. 



neph Nephridia. 



neu.p Neuropore. 



n.l Neural lamina of cephalic shield. 



n.n Neural nerve. 



noto Notochord. 



n.p Neuropile; neural process; neural plate; neuropore. 



np.d .Nephric duct. 



n.r Nerve ring. 



n.st Primitive mouth; neurostoma 



n.tr Neural tracts. 



n.v Neural vessel. 



o.b Opening to branchial chamber. 



oc Ocelli; occipital plate. 



o.c Occipital line of canal organs. 



oc.f Occipital foramen 



od Oviduct. 



oe (Esophagus. 



ol Primitive olfactory organ; frontal organ. 



ol.l Olfactory lobes. 



ol.l.n Lateral nerve of olfactory organ. 



ol.m.n Median nerve of olfactory organ. 



ol.np Olfactory neuropile. 



ol.o Olfactory organ. 



ol.r . . Olfactory recess. 



ol.v Ventricle of olfactory lobes. 



o.m Circumoral membrane. 



op Operculum. 



o.p Postorbital valley. 



op.g Optic ganglion. 



op.neu Optic neurones. 



op.pl Optic plate; opercular pleurite. 



op.s Opercular segment. 



op.tr Optic tracts. 



