MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 
433 
Plate LXXXYI — Continued. 
Area ( Argina ) pexata : 
Fig. 50. Single gill filament, il, inner lamella ; ol, outer lamella; ife, interfilamentar connection. 
Cardita borealis : 
Fig. 51. Vertical, transverse section in region of pedal ganglion; s, stomach; l, liver; <j, generative 
mass; ig, inner gill; pp, posterior palp; me, mantle edge; pg, pedal ganglion, with cerebral 
commissure running dorsally. 
Yoldia limatula : 
Plate LXXXVII. 
Fig. 52. Removed from shell ; right mantle fold dissected off ; aa, anterior adductor ; pa, posterior 
adductor ; l, liver mass and generative gland, on the surface of which may be seen the loop of 
the intestine, i; p, large palp, extending beneath visceral mass. At its posterior end springs 
the appendage ap; g, gill, extending from posterior end of palp to base of siphons, s; me, 
mantle edge ; /, foot ; d, ventral disc of foot. 
Fig. 53. Transverse section across siphon of Yoldia, showing portion of wall of anal, as, and bran- 
chial siphons, br, including a portion of siphonal septum, ss; e, covering of exterior of 
siphons ; it is more or less transparent, contains numerous nuclei, and, at places, indications 
of elongated cell boundaries ; trm, regularly arranged bundles of transverse muscles in walls 
of both siphons and siphonal septum ; Im, numerous bundles of longitudinal muscles (here 
cut transversely), alternating regularly with the transverse layers. 
Ostrea virginiana — Oyster : 
Fig. 54. Portion of secretory epithelium of nephridium in Ostrea; exe, excretory cells, with deeply 
stained bases and transparent globular free ends ; l, lumen of excretory tube, into which 
break off fluid globular ends of cells. Many of these contain a small stained body. 
Macira solidissima : 
Fig. 55. Crystalline style. End teased off in spiral from softer central axis, ca. 
Pecten ir radians — Scallop : 
Figs. 56 and 57. Concretions showing concentric structure, from excretory cells of nephridium. 
Fig. 58. Excretory cell from nephridium; con, distally placed concretion (these all show a deeply 
stained central portion of various shapes and sizes); vac, numerous small vacuoles; nuc 
nucleus. 
Fig. 59. Elongated cells from same, containing numerous spherical vacuoles of varying sizes. 
Fig. 60. Segmenting eggs found in nephridium. 
Solenomya velum : 
Fig. 61. Single siphonal opening in mantle. A, walls not bent; B, walls bent to form upper, as, 
and lower siphonal openings, bs; t, tentacles. 
Ostrea virginiana — Oyster: 
Fig. 62. Transverse section of palp of Ostrea, near ventral or outer edge. Inner surface thrown 
into ciliated ridges or folds, /. In this region these consist of two secondary folds, s/; ct, 
irregular membrane at base of folds; fc, fat cells. 
Fig. 63. Same, midway between ventral edge and base, showing difference in character of folds, /. 
Fig. 64. Same, at base. No secondary folds. Supporting tissue at base of folds much thicker, ct; 
by, blood vessel. 
Plate LXXXVIII. 
Ostrea virginiana — Oyster : 
Fig. 65. Striated muscle fibers in auricle of heart ; smf, striated muscle fibers, generally, if not 
always, attached to thick, homogeneous, supporting membrane of wall, sm; a, protrusion 
of supporting membrane through epithelium of wall to exterior; pgc, pigment cells; v, 
vacuolated epithelial cells scattered throughout muscle fibers. 
Area {Argina) pexata: 
Fig. 66. Cross-section of gill filaments; glc, gland cells; If l, cilia of straining line; eh, chitin; 
cj, ciliated junction; /, frontal epithelium; vs, vascular space between walls connecting 
lower part of descending and ascending filaments. This filament was cut lower down than 
the other three represented in the figure (see Fig. 50). 
F. C. B. 1890—2* 
