34 



COMPARATIVE ANATOMY 



CHAP. 



D. Seaphopoda. 



The body is symmetrical and long, i.e. the visceral sac is elongated 

 dorso-ventrally, and is completely enveloped in a tubular mantle. The 

 mantle cavity lies posteriorly, and is prolonged ventrally far enough to 

 allow the snout and retracted foot to be completely concealed in it. 

 Besides the large ventral aperture, there is a smaller dorsal aperture 

 further placing the mantle cavity in communication with the exterior. 

 The shell, like the mantle, is tubular, or like a tapering cone, slightly 

 curved anteriorly. It has two apertures corresponding with those in 

 the mantle. The head is developed into a barrel-shaped snout, and 

 has no eyes. The mouth, which lies at its ventral end, is surrounded 

 by a circle of leaf-like tentacles. At the base of the snout there arise 

 two tassels of long filamentous contractile tentacles, which hang down 

 into the mantle cavity and can be projected far beyond the ventral aper- 

 ture. Behind the snout, the cylindrical muscular foot rises from the 

 body, and can be protruded downwards. There are no gills. The 



median anus lies posteriorly above 

 the foot. The two nephridial 

 apertures are at the sides of the 

 anus. There are no special genital 

 apertures (Figs. 44 and 101, 

 p. 113). 



E. Lamellibranehia. 



The body is bilaterally sym- 

 metrical ; somewhat elongated 

 (from before backward). The 

 integument forms leaf-like mantle 

 folds to the right and to the left, 

 which at their bases are attached 

 to the trunk along its whole 

 length, and grow down ventrally. 

 If the body of a Lamellibranch, 

 from which the shell has been 

 removed (the foot being re- 

 tracted), be viewed from the side, 

 the outline will be found to be 

 formed, dorsally, by the dorsal 

 median line of the body ; an- 

 teriorly, posteriorly, and ventrally 

 by the free edge of the mantle 

 fold. The two mantle folds en- 



FiG. 45. — Transverse section of Anodonta 

 cygnsea {ordinary fresh water mussel) (after Howes). 

 Ig, Ligament ; (i/, typhlosolis ; ki, pericardial gland 

 (Keber's organ) ; re, kidney (glandular portion) ; 

 sic, chambers at the bases of the gills ; gd, genital 

 ducts ; hrli, brl^, outer and inner branchial lamellic ; 

 ibc, mantle cavity ; s, shell ; sj, edge of the shell ; 

 /l. foot ; 3>iii, pallial muscle ; i, intestine ; plj, right 

 mantle fold ; ggl, gonad ; r, rectum ; cp, cerebro- 

 pedal connective ; rei, non-glandular vestibule of 

 kidney ; rej, renal aperture ; jjc, pericardium. 



close a space whose transverse 

 axis ^ IS always markedly shorter than either its dorso-ventral or its 

 longitudinal axis, i.e. the animal with its mantle is laterally compressed. 



