HORNELL- ANATOMY OF PLACUNA 95 



Fig. 5. Variations in the form of anal funnel in four individuals. X 5. 



6. Diagrammatic representation of the two nephridia in plan, showing the unequal development 

 of the posterior caeca (R.n.c and L.n.c), the internephrklial passage into which the genital 

 duct opens (Go.a), and the relation of the ventral visceral \abe (V.v.l) which appears to 

 pierce the nephridial sac. Acc.ex.o, accessory excretory organ ; Rn.o, external renal 

 apertures; C.st, crystalline style; Ao.v, ventral branch of aorta. 



7. Lateral view of the right nephridium. 



,, 8. Lateral view of the left nephridium. 



I.n.p, marks the situation of the intcrnephridial passage. 



,, 9. Insertion surface of the adductor muscle seen from left side. Add', anterior region ; Add", 

 posterior region. 



,, 10. Plan of the nervous system, showing the ganglionic centres, connectives, and principal nerve 

 trunks. For lettering consult the list on p. 93. 



,, 11. Nervous system dissected out and seen from the left side. S.o, pallial sense organ; Add, 

 adductor muscle : 0, mouth; Oe, oesophagus. Other lettering as in Fig. 10. 



1 2. Diagrammatic transverse section across the two ctenidia to show the relative arrangement of 

 lamella; and vessels. Jir.ms, branchial mesentery ; Br.aff. and Jir. efl', afferent and efferent 

 branchial vessels respectively; Jir. ace, accessory fold of the reflected outer lamella 

 (R.br.l) ; D.br.l, inner and outer direct branchial lamellae ; Ct.r and Ct.l, right and left 

 ctenidia; Ct.j, junction between inner reflected lamellie of right and left ctenidia. 



PLATE III. 



1 3. Dissection of Placuna placenta, from the left side to show the course and situation of the 



digestive system, position of heart, and other details. The left mantle, the whole of the 

 branchiae, the left nephridium, the left palp and the superficial portion (left side) of the 

 visceral mass have been removed. The foot is seen in moderate extension. 8.0, right 

 pallial sense organ ; Sn.pall.a.v, anterior ventral pallial sinus of right mantle ; C.st, 

 crystalline style. Natural size of a two years old individual. 



14. Dissection of the alimentary canal, natural size. 



15. Outline sketch of a dissection of a two years old individual to show the course of the blood 



vessels. The vessels of the arterial system are shown by means of double lines, while the 

 venous sinuses are shown in heavy black. The outline of the branchiae is shown by means 

 of a dotted line. Natural size. For explanation of lettering see list on p. 93. 



16. Diagram of heart seen from left side. 



1 7. right side. 



18. Diagram of an abnormal heart found in an individual from Tuticorin ; in this case the right 



auricle, does not communicate with the ventricle. Compare with Fig. 1 7 where the normal 

 arrangement is depicted. 

 Lettering for these figures : 



V, ventricle; Ait.t, left auricle; Au.r, right auricle; Au.ch, inter-auricular channel; Ao, 

 aorta ; Ao.v, ventral brancli of same ; Sn.r, renal sinus. 



1.9. Left pallial lobe viewed from without, to show the elaborate fan-like branching of the pallial 

 muscles and their insertion centres (M.ins.a and M.ins.p), anterior and posterior to the 

 adductor muscle (Add), also the . course of the pallial blood sinuses (Sn.pall). These 

 branched vessels are distinguished by means of double lines. Lev, insertion surface of 

 pedal levator muscle. 



