432 lin.LETIN OF THE UNIThD STATES FISH COMMISSION. 



Plate LXXXIV. 

 Mijlihis idiilis — Oommun mussel: 



Fui. 33. TiMQsverso, vertical sectiou iu region of luoatli, hid; I, liver; ol, outer lamella of outer fj;ill; 



il, iuuer lamella of iuuergill; m, thicTcened mautle, coutaiuiug sexual glaml; pi, auterior 



palp; me, mantle edge, joined anteriorly {m) down to a position almost beneath the month ; 



ar, anterior retractor of foot; /', foot extending forward and flattened dorso-ventrally. 

 Fi<;. 34. Same in region of stomach, s; p. anterior end of pericardium; I, liver; (/, generative mass. 



(The visceral mass is seen to be relatively very small. ; 

 Fici. 35. Same, in region just anterior to base of foot; h, anterior end of heart; w, water tube 



between gill lamelhv, ; 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. 

 Fk;. 36. Same, in region of l)asc of foot, /'.• fm, posterior retractors of foot, running directly upward 



to liecome 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, 6; /(, heart, containing rectum; /• (auricles 



indicated at sides) ; hm, byssus muscles attached to shell and combining to support byssus 



organ. 

 Fig. 38. Same, in region of posterior end of heart, h. 



Flatk LXXXV. 



Mfitiluff edulis — CommuH mussel: 



Fiti. 39. Vertical, transverse section just in front of posterior adductor; bin, most posterior of 

 byssus muscles; ab, posterior region of abdomen or visceral mass; /•, rectum. 



Fig. 40. Same, in region of anterior end of posterior addnctor; /, intestine; r, rectum; y;«. poste- 

 rioi adductor; ub, extreme posterior end of visceral mass. 



Fi«;. 41. Same, in region of posterior end of posterior adductor. 



Fig. 42. Side view of chief mus(de systeui of Mi/fUii.'-i: na. anterior ailductor ; pa, posterior adductor ; 

 (tr, anterior foot-retractors, running i)ackward between posterior foot-retractors, or muscles 

 suspending the foot, /»H. and joining the l>yssus 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; ft »i. byssus muscles; 

 bf/s, byssus. 

 Fecten ir radians — Scallop : 



Fig. 43. Vertical, transverse section in region of mouth; /, liver; mo, large mouth opening iiit() 

 very wide (esophagus; ap, anterior palp; fr, part of fringe of anterior lip. the median por- 

 tion of the two anterior palps. 



Fi<i. 44. Same, in region of stomach, s, the walls of which :ire extremely uneven and folded; i, 

 intestine. 



Plate LXXXVI. 

 Pecten irradiaiis — Scallop: 



Fig. 4.5. Vertical, transverse section in regi<m of heart; ^, large pericardium; no. opening at base 

 of pericardium into the large nephridium, n, below ; /, intestine ; g, generative mass ; a, por- 

 tion of the adductor muscle; ven, A^entricle of heart with rectum, )•, attached to its dorsal 

 wall ; ail, 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 eontracfed dark 

 portion, da, and mantle edges, me, not fused above. 

 Yoldia Umatilla: 



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 ; mf. muscle fibers, occasionally 

 showing nuclei, in interior of plate; lac, lacunar tissue. 

 yiytilus edulis — Common mussel : 



Fig. 49. IJiagrim of laraellie of two gills; bv, blooil vessel in ends of outer lamella, continuous 

 throughout all filaments; (7c, interlamellar connection; i/c, ciliated, interfilamentar con- 

 nections. 



