Millimeters 



FiGTRE 121. — Transverse section of the demibranch of C. viriginica. Kahle fixation, hematoxylin-eosin. ch.r. — chitinous 

 rods; g. — groove; if.j. — interfilamentar junction; il.m. — interlamellar muscles; il.s. — interlamellar septum; l.m. — 

 longitudinal muscles of the interlamellar septum; o. — ostium; o.f. — ordinary filament; p.f. — principal filament; 

 pi. — plica; t.f. — transitional filament; tr.m. — transverse muscle of the interlamellar septum; w.t. — water tube. 



content to tlio surface of the gills. The secreted 

 mucus spreads over tlie gill plates and entangles 

 the particles which settle on them. At the distal 

 edge of the gill the filaments are fused together 

 to form a terminal groove along wliich food is 

 conveyed toward tlie mouth. The principal 

 filaments differ from the others by their larger 

 size and nontubular appearance. At tlie base of 

 each filament there is a blood vessel located in a 

 space between tlie chitinous rods. The epithelial 

 cells are slightly larger than those of the ordinary 

 filaments and have longer cilia (fig. 126). 



OSTIA 



Ostia or fenestrae, the oval-shaped open spaces 

 between two adjoining filaments (figs. 121 and 125, 

 o.), are framed by two vertical and two horizontal 



skeleton bars covered witli epithelium. Their 

 configurations and dimensions vary, depending 

 on the degree of contraction of the filamental 

 musculature and the distention of blood spaces. 

 In an actively feeding oyster the contraction and 

 expansion of the ostia regulate the passage of water 

 through tlie gills. This can be observed on the 

 surface of a gill exposed by cutting off a piece of 

 shell. The dimensions of the ostia are somewhat 

 correlated to the size of the oyster ova which pass 

 through the gills during spawning (see ch. XIV, 

 p. 303). In a viviparous 0. lurida the ostia are 

 large, varying from 90 by 45 /u in contracted 

 state, to ISO by 60 n when fully expanded. The 

 ova of this species average 90 m in diameter. In 

 C. gigas and C. virginica, which have smaller eggs, 



THE GILLS 



127 



