EXPLANATION OF PLATES. 



INDEX TO REFERENCE LETTERS. 



A.ch, afferent blood channel of gill filament. 



A.l.c, antero-lateral cilia of gill filament. 



Acc.ex.o, accessory excretory organ. 



Add, adductor muscle. 



Add', anterior region of adductor muscle. 



Add", posterior ,, 



An.f, anal funnel. 



Ao, aorta. 



Ao.v, ventral trunk of aorta. 



Au.ch, interauricular channel. 



Au.l, left auricle. 



Au.r, right auricle. 



Art.add, dorsal arteries of adductor muscle. 



Art.add', ventral arteries of 



Art.a.ji, right anterior pallial artery. 



Art.a.p, left 



Art.c.a, anterior common pallial artery. 



Art.c.p, posterior ,, ,, 



Art.hep, hepatic artery. 



Art.pa, palpar artery. 



Art.p.p, right posterior pallial artery. 



Art.p.p, left 



Art.visc, visceral artery. 



Br, branchia. 



Br.acc, accessory fold of the reflected outer 



branchial lamella. 

 Br.eff, efferent branchial vessel. 

 Br.l, left branchia. 

 Br.rms, branchial mesentery. 

 Br.iius.pl, plexus of blood capillaries in branchial 



mesentery. 

 Br.r, right branchia. 

 Br.ret, branchial muscle band. 

 C.br.aff, common afferent branchial vessel. 

 C.br.aff', left common afferent branchial vessel. 

 C.br.aff", right 



C.br.eff, common efferent branchial vessel. 



C.p.coii,, cerebro-pedal connective. 



C.st, crystalline style. 



C.st.c, c;ecum. 



C.t, cardinal teeth. 



C.v.coit, cerebro-visceral connective. 



Cer.com, cerebral commissure. 



C'er.g, cerebral ganglion. 



Cer.g.l, left cerebral ganglion. 



Ger.g.r, right 



D.(jl, digestive gland. 



D.tjl.d, ducts. 



E.ch, efferent blood channel of gill filament. 



E.ejj, epithelium of outer pallial surface. 



/', foot. 



A'.c, frontal cilia on outer face of a gill filament. 



fl.tri, fleche tricuspide. 



Go, gonad. 



Go.a, external aperture of gouad. 



Go.p, posterial genital lobe. 



I.n.p, iuternephridial passage. 



I.ep, epithelium of inner pallial surface. 



Int, intestine. 



Int. 1, first or ascending limb of intestine. 



Int. 2, second or descending limb of intestine. 



L.c, lateral cilia of a gill filament. 



L.n.c, left nephridial caecum. 



Lev, pedal levator muscle. 



Lev, insertion surface of levator muscle. 



M.ins.a, insertion surfaces of the anterior pallial 



muscles. 

 M.ins.p, the principal insertion of the posterior 



pallial muscles. 

 Jfy.jt, pallial margin. 

 Jfg.v, velar margin. 

 ^V, nerve. 



