WOODWAED : ANATOMY OF MtTLLEEIA DALTI. 89 



the adductor this chamber becomes divided into three by the union of 

 the primary branchial filaments with the body- wall on either side. 

 Still further forward, the middle of these three chambers becomes 

 divided by the intervention of the visceral mass. In the relationship 

 of this cavity to the surrounding parts, MuUeria resembles typical 

 Eulamellibranchiata ; there is, however, no communication, other than 

 through the gill-slits, between the branchial and supra-branchial 

 cavities, such as we meet with in a form like Anodonia. 



The external apertures of the organ of Bojanus {r.o.), and of the 

 genital gland, open into the innermost subdivision of the supra- 

 branchial cavity. 



The ^oot and Visceral Mass. — The muscular foot present in most 

 lamellibranchiata is entirely wanting in Mulhria, a condition which 

 is correlated with the fixed mode of life of this form. JEtJieria, on 

 the other hand, although fixed, is stated to possess a large, thick, and 

 powerful foot.^ This form, however, still retains the two adductor 

 muscles, and is evidently less modified than MiiUeria. 



The visceral mass is small and slightly bilobed posteriorly; it 

 exhibits a few superficial muscular fibres mainly connected with the 

 posterior pedal retractor. 



The Labial Palps. — Two pairs of very small leaf-shaped labial palps 

 are present ; they are equal in size, and exhibit oblique ridges on the 

 apposed surfaces. The inner pair unite below the mouth to form the 

 lower lip, while the outer pair, which become slightly expanded 

 anteriorly, unite above the mouth. 



The Respiratory and Circulatory Systems. — The gills of MuUeria 

 consist of two lamellae on either side of the body ; of these the outer 

 is slightly the smaller. Their relations to each other, to the mantle 



Fig. II. — Section across the gill lamella, cutting the gill filaments {g.f.) 

 transversely ; b.v., blood-vessel ; i-f-j-, interfilamental 

 junctions; i.l.j., in terlamellar junctions. 

 ,, III. — Transverse section across two gill filaments ; s.r. , supporting rods. 



1 An examination of Eang & Caillaud's figure has caused some doubt to arise in 

 my mind, whether ^theria really possesses a muscular foot distinct from the visceral 

 mass. S. P. Woodward, in his Manual of Mollusca, states (p. 276 ; 4th ed., p. 435) 

 that there is no trace of a foot. 



