BULLETIN OF THE UNITED STATES FISH COMMISSION. 
4o2 
Plate LXXXIY. 
Mytilus edulis — Commo-n mussel : 
Fig. 33. Transverse, vertical section in. region of mouth., mo; l, liver; ol, outer lamella of outer gill; 
il, iuuer lamella of iuuer gill; m, thickened mautle, coutaining sexual gland; pi, anterior 
palp; me, mantle edge, joined anteriorly (m) down to a position almost beneath the mouth; 
ar, anterior retractor of foot; /, foot extending forward and flattened dorso-ventrally. 
Fig. 34. Same in region of stomach, s; p, anterior end of pericardium ; l, liver; g, generative mass. 
(The visceral mass is seen to be relatively very small. ) 
Fig. 35. Same, in region just anterior to base of foot; h, anterior end of heart; to, water tube 
between gill lamellae ; ar, anterior foot-retractors close together, and in the following figures 
are seen to run between the only retractor muscles of the foot, fin; i, intestine. 
Fig. 36. Same, in region of base of foot, f; fm, posterior retractors of foot, running directly upward 
to become attached to shell ; ar, anterior-foot retractors, running between posterior retractors 
to become attached to byssus organ. 
Fig. 37. Same, in region of byssus organ and byssus, b; h, heart, containing rectum; r (auricles 
indicated at sides) ; bm, byssus muscles attached to shell and combining to support byssus 
organ. 
Fig. 38. Same, in region of posterior end of heart, h. 
Plate LXXXY. • 
Mytilus edulis — Common mussel: 
Fig. 39. Yertical, transverse section just in front of posterior adductor; bin, most posterior of 
byssus muscles; ab, posterior region of abdomen or visceral mass; r, rectum. 
Fig. 40. Same, in region of anterior end of posterior adductor ; i, intestine ; r, rectum ; pa, poste- 
rior adductor ; ab, extreme posterior end of viseeral mass. 
Fig. 41. Same, in region of posterior end of posterior adductor. 
Fig. 42. Side view of chief muscle system of Mytilus; aa, anterior adductor ; pa, posterior adductor ; 
ar, anterior foot-retractors, running backward between posterior foot-retractors, or muscles 
suspending the foot, fm, and joining the byssus organ. The muscles of the foot itself are 
entirely free from any connection with these. This organ, /, and its retractors, fm, may be 
removed from all the other muscles without anywhere injuring them; bin, byssus muscles ; 
bys, byssus. 
Pecten ir radians — Scallop : 
Fig. 43. Yertical, transverse section in region of mouth; l, liver; mo, large mouth opening into 
very wide oesophagus; op, anterior palp; fr, part of fringe of anterior lip, the median por- 
tion of the two anterior palps. 
Fig. 44. Same, in region of stomach, s, the walls of which are extremely uneven and folded; i, 
intestine. 
Plate LXXXVI. 
Pecten irradians — Scallop: 
Fig. 45. Vertical, transverse section in region of heart ; p, large pericardium; no, opening at base 
of pericardium into the large nephridium, n, below ; i, intestine; g, generative mass ; a, por- 
tion of the adductor muscle ; ven, ventricle of heart with rectum, r, attached to its dorsal 
wall ; aa, auricle of heart running downward to connect with large vein from gills, gl. 
Fig. 46. Same, in middle region of adductor, da, small, closely contracted portion of adductor with 
dark, striated fibers; a, main portion of adductor, composed of white fibers; gm, membrane 
suspending gills. 
Fig. 47. Same, upper, posterior portion of adductor, showing position of greatly contracted dark 
portion, da, and mantle edges, me, not fused above. 
Yoldia limalula: 
Fig. 48. Horizontal section through inner edges of gill plates in Yoldia (Fig. 81, d ) ; ml, median 
line ; a, space in interior of a plate ; b, space between plates ; inf, muscle fibers, occasionally 
showing nuclei, in interior of plate ; lac, lacunar tissue. 
Mytilus edulis — Common mussel : 
Fig. 49. Diagram of lamellae of two gills; be, blood vessel in ends of outer lamella, continuous 
throughout all filaments; ilc, interlamellar connection; ife, ciliated, interfilamentar con- 
nections. 
