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OKHAMANDAL MARINE ZOOLOGY REPORT 



N.add, adductor nerve. 



N.a.p, anterior pallia] nerve trunk. 



N.br, branchial nerve. 



N.pall, pallial nerves. 



N.s.o, nerve to pallial sense organ. 



N.v, visceral nerves. 



N.v.v, ventral visceral nerves. 



Neph.l, left nephridium. 



Neph.r, right nephridium. 



0, mouth. 



Oe, oesophagus. 



Pa, palps. 



Fall, pallium or mantle. 



Pall.l, left pallial lobe. 



Pall.r, right ,, „ 



Par.sp.ff, parieto-splanchnic ganglion. 



Ped.g, pedal ganglion. 



Periost, periostracum. 



R, rectum. 



R.n.c, right nephridial csecum. 



Ret, retractor muscle of foot. 



Rn.o, renal orifice. 



S.o, pallial sense organ. 



Sep, transverse septum of gill filament. 



Sk.h, skeletal band within a gill filament. 



Sn, blood sinus. 



Sn.m, median sinus. 



Sii.j}, pedal sinus. 



Sn.2}.o, opening of pallial sinuses into common 

 efferent branchial vessels. 



Sn.jj.v, branches of posterior visceral sinus. 



Sn.pall.a.v, anterior ventral pallial sinus. 



Sn.r, rectal sinus. 



Sn.m, renal sinus. 



Sn.v.l, left visceral sinus. 



Sn.v.r, right „ „ 



St, stomach. 



V, ventricle. 



V.ao, aortic valve. 



V.a.-v, auriculo-ventricular valves. 



V.fft, gutter along ventral surface of rectal 

 genital lobe. 



V.v.l, ventral visceral lobe. 



Vel, velum or velar margin. 



X, left branchiae cut across, posterior half re- 

 moved. 



X', mantle cut away along this line. 



PLATE I. 



Fig. 1. Young window-pane oysters (P. placenta) of three ages, all seen from left side. The youngest 

 is probably not more than six weeks old, the three oldest three to four months. Note that 

 all the latter show well-marked " ears " ; as many as three anterior ones are shown by the 

 one on the left, bottom row. X |. 

 ,, 2. Two adult individuals, two to two and a half years old, seen from the right side, showing 

 extensive injury caused by fish-bites, recent as well as of old standing. All injuries in 

 process of reparation. Fig. B shows the remains of a posterior " ear " of considerable size ; 



Fig. A shows a well-marked straight hinge line. X 



■s^- 



PLATE II. 



Fig. ] . Inner aspect of the hinge region of the right valve of Placuna placenta ; note the two long 

 cardinal teeth of which the posterior is the longer, and the long row of short hinge teeth 

 along the lower edge of the ligament (I). 



Fig 2 and 3. Similar views from within of the lunge region of two left valves, showing the high 

 variability of the hinge dentition. In these views the two long divergent black bands 

 (Let), represent the ligaments of the cardinal teeth belonging to tlie right valve. I, hinge 

 ligament with dentated margin corresponding with the row of hinge teeth on the opposing 

 valve, a.e and p.e, the anterior and posterior " ears '' respectively of the shell. 



Fig. 4. General anatomy of Placuna as actually displayed in a dissection ; the animal is viewed from 

 the left side, the left mantle lobe and the distal half of the left ctenidium being removed 

 along the lines marked X' and X. Actual size of a two years old individual. 

 For explanation of the reference letters, see list above. 



