54 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 



EXPLA^^ATION" OF THE PLATE. 



All the figures are magnified, with the exception of Figs. 1 and 2. 



Fig. I. Cyclas {Sphcerium) with foot extended : a, shell ; 6, foot ; all natu- 

 ral size. 



Fig. II. Respiratory tubes extended : a, valves ; 6, tubes ; all natural size. 



Fig. III. Cyclas (Sphcerium) seen from below : a, valves ; b, foot ; c, posterior 

 transversal membrane ; d, interior pair of gills ; e, exterior pair of gills. 



Fig. IV. Animal, valves removed in order to show the mantle : aab c, the 

 mantle : a a, dorsal part ; b c, inferior part open ; d e, the edges ; /, anterior 

 adductor muscle ; g, posterior adductor ; h, crescent-shaped membrane in the 

 anterior angle of the mantle opening ; i, membrane in the posterior angle, with 

 its two apertures and k its transverse muscle ; I I, crescent-shaped lips formed by 

 the edge of the mantle on each of its sides ; m, continuation of edges running 

 up towards the dorsal region ; n, portion of the heart ; o, intestine running 

 through the heart ; p, position and outline of the liver. 



Fig. v. The transverse membrane e.xtended : a, upper aperture, leading to 

 the gill-canal ; b, inferior aperture, leading to the cavity of the mantle ; 

 c, filaments encircling the apertures. 



Fig. VI. Foot and abdomen : a, foot contracted ; b, ligaments holding it ; 

 c, mouth ; d, tentacula of mouth ; e, liver ; /, first curve of the intestine ; 

 g, portion of the intestine running through the ovaries ; h, rectum ; i, ovaries 

 and the small cylinders composing them ; a, /3, v, nervous system ; a, anterior 

 ganglion ; v, posterior ganglion. 



Fig. VII. Posterior and relative size of the gills : a b, exterior gill ; c d, in- 

 terior gill ; e, gill-sac. 



Fig. VIII. Interior gill and gill-sac : a, foot ; b, mouth ; c, tentacula of mouth ; 

 d e, interior gill lifted up ; /, interior membrane of gill-sac ; g, the large crescent- 

 shaped aperture ; h, posterior end of interior gill. 



Fig. IX. Section showing the gill-sac : a, mantle ; b, exterior gill with its 

 small gill-canal ; d, interior gill, with its gill-sac ; e, exterior membrane ; /, in- 

 terior membrane ; g, ovaries ; h, capsules ; i, young. 



Fig. X. A pair of pear-shaped capsules. 



Fig. XI. Young at different stages, considerably magnified : a, valves ; 

 6, protruding foot. 



