MOKPHOLOGY OF LAMEL LIRE ANC HI ATE MOLLUSKS. 
431 
Plate LXXXII — Continued. 
Venus mereenaria — Quahog — Continued. 
Fig. 18. Same, in region of middle of posterior adductor, pa; to, thickened muscular mantle edge ; 
tvs, lower wall of branchial siphon seen posterior to branchial membrane, brm. (This repre- 
sents the anterior surface of the section. 1 
Fig. 19. Same, in region of base of siphon, us, upper or anal siphon; Is, lower or branchial siphon : 
x, point where branchial siphon remains continuous with mantle; to, mantle continued 
across below siphons ; ss, siphonal septum. 
Fig. 20. Same, just posterior to last. Siphons still beneath posterior adductor, pa, separated from 
mantle, which is yet continuous beneath them. 
Fig. 21. Same, posterior to last, mantle not continuous beneath siphons. Lumen of branchial 
siphon, Is, assumes oval outline. 
Fig. 22. Same, through free siphons. 
Mya arenaria — Long clam : 
Fig. 23. Vertical, transverse section in region of anterior adductor, a; me, thick mantle edge; c, 
cuticle from mantle edge ; to, mantle below adductor ; branchial chamber closed in by con- 
crescence of mantle edges below ; /, foot much flattened laterally. 
Plate LXXXIII. 
Mya arenaria — Long clam : 
Fig. 24. Vertical, transverse section in region of anterior end of visceral mass; me, mantle edge; 
oe, thick- walled (esophagus ; op, outer palp ; ip, inner palp; to, mantle; g, generative mass ; 
/, liver. 
Fig. 25. Same, in region of anterior end of stomach, s ; ig, inner gill ; og, outer gill ; eo, epibranchial 
chamber; i, intestine; cst, crystalline style running forward on ventral side of abdomen. 
Fig. 26. Same, in region of posterior end of stomach; bv. blood vessel; cst, crystalline style, orig- 
inating from ventral wall of stomach, s, and running on right side of body to bottom of 
visceral mass. It then turns forward, and is shown in section in the preceding figure. 
Fig. 27. Same, in region of heart, h; n , uephidium; ol, outer lamella of outer gill ; to/, muscle fibers 
in generative gland. 
Fig. 28. Diagram to show plane of sections through folded gills of Mya and Venus, explaining 
reason of folded appearance in vertical sections of the whole animal. 
Fig. 29. Section just anterior to base of siphons, the anterior face of section; r, rectum, below 
plane of section, opening into continuation of epibranchial or the cloacal chamber, cl; gl, 
outer lamella, of gill in concrescence with mantle. Gills here form complete septum between 
branchial and epibranchial chambers; 6s, base of branchial siphon below plane of section; 
ws, siphonal walls; brm, fold at anterior end of siphonal septum, probably representing 
branchial membrane of Venus, Maetra, etc.; el, jelly-like layer, clothing posterior end of 
mantle and extending out over siphons. 
Fig. 30. Same, through base of siphons, right side cut a little deeper than left. The thickened 
mantle walls, ws, in the previous figures, become here the siphonal walls, ws, and are now 
much thicker; el, upper end of cloaca opening below into anal siphon, us; ss, siphonal 
septum. 
* Fig. 31. Same, through siphons at a distance from mantle; bv, blood vessel. A similar vessel is 
present above the anal siphon. 
Mytilus edulis — Common mussel : 
Fig. 32. Transverse, vertical section through anterior end of body ; l, liver mass ; og, outer gill ; ig, 
inner gill ; to, thickened mantle, containing sexual gland. That portion on the median line 
has been drawn in greatly on killing, and thus appears in section ; ar, anterior retractor ; 
pi, palp ; aa, anterior adductor ; me, mantle edge. 
