Centimelers 



Figure 119. — Cast of the water exhalant system of the 

 gill of C. gigas viewed from the left side. cl. — cloaca; 

 l.d. — left deniibranch; pm. eh. — promyal chamber; 

 r.d. — right demibranch. 



the accumulation of sediment in the paihal cavity. 

 No evidence has 3-et been presented to corroborate 

 this view. The physiological significance of the 

 chamber is not known beyond the fact that it pro- 

 vides an additional outlet for the discharge of 

 water through the gills. Elsey (1935) remarks 

 that in C. gicjas the promyal chamber allows a free 

 passage of water when the gonads distend before 

 spawning and encroach into the branchial space, 

 partially closing the water tubes. Sections of the 

 visceral mass of C. virginica with fully developed 

 gonads do not show any "encroachments" into 

 branchial space. Tlie latter remains unob- 

 structed, and the water tubes fully open. At 

 present it is difficult to see what structural and 

 physiological advantages are gained by the pres- 

 ence of a promyal chamber in Crassostrea oysters. 



GILL LAMELLA 



Each lamella of a gill consists of a great number 

 of tubular filaments arranged at right angles to the 

 axis of the gill. At the edge of the plate the fila- 

 ments are reflected on themselves and continue 



upward along the plane of the ascending lamella. 

 The filaments forming the gill-lamella do not lie 

 smoothly on one plane; they are arranged in a 

 series of transverse folds or plicae that give the 

 surface of the gill a plaited appearance noticeable 

 to the naked-eye. A transverse section (fig. 121) 

 shows the arrangement of filaments in alternating 

 grooves and ridges. The number of filaments on 

 a single fold of an adult C. virginica is not constant. 

 In my preparations it varied from 10 to 16 per 

 fold. 



There are three types of filaments that can be 

 distinguished by their position, shape, and dimen- 

 sions. The larger, or principal filaments (fig. 121, 

 p.f.), are located at the bottom of the groove 

 between the plicae. In cross section they have a 

 triangular shape with two bulky chitinous rods 

 forming two sides of this triangle. The rods are 

 fused at the apex but are separated at the base, 

 which contains a narrow blood vessel. The two 

 transitional filaments (t.f.), one on each side of 

 the principal one, are smaller and diff'er in shape 

 from the ordinary filaments (o.f.), which form the 

 rest of the plica. Sometimes the difll'erence is 

 insignificant. In general the ordinary filaments 

 seen on cross section are elongated, club-shaped 

 units. 



Throughout their length the filaments of each 

 plica are joined at the bases by regularly spaced 

 interfilamentar junctions (if.j.), which consist of 

 narrow bands of vascular connective tissue. The 

 free portions of the filaments surround the oval- 

 shaped openings, called ostia or fenestrae (o.), 

 through which the water enters the inside passages 

 of the gill. Numerous muscle fibers follow the 

 interfilamentar junction and extend along both 

 sides of the plica to its distal part (if.m.). At 

 intervals, the two lamellae of each demibranch 

 are connected by the partitions (interlamellar 

 septa) made of connective tissue (il.s.) which run 

 across the plate from one lamella to the otiier. 

 These partitions are more numerous toward the 

 distal (free) edge of the demibranch and diminish 

 toward the base of the gill, where they are found 

 only at about every sixth plica. The interlamellar 

 junctions contain numerous muscle fibers, blood 

 vessels, and nerves. The muscle fibers are 

 arranged in three systems: the longitudinal mus- 

 cles (l.m.) seen in cross section extend vertically 

 from the proximal to tlie distal end of the lamellae; 

 the transverse muscles (tr.m.) go from one side of 

 the lamellae to the other; and the tangential 



THE GILLS 



73S-851 0—64- 



125 



