Microns 



20 



Figure 127. — Cross section of the filament of the gill of 

 0. eduUs, according to Atkins, 1938. an.l.fc. — anterior 

 laterofrontal cilia; c.f.c. — central frontal cilia; f.f.c. — 

 fine frontal cilia; para l.fc. — paralaterof rental cilia; 

 m.g. — mucous gland; 1. — lateral cilia; t.m. — transverse 

 muscle fiber. Chitinous rods are shown as black areas 

 under the epithelium. 



branchs or only a group of filaments stand apart 

 and are constricted when the latter are drawn 

 together. 



Changes in the position of the demibranchs de- 

 pend on two distinct systems of muscles located at 

 the gill axis above and below the skeletal arches. 

 In general the muscle fibers follow the configura- 

 tion of the arches. The larger bands located in- 

 side the arches are the flexor muscles, which are 

 attached to the inner sides of the two arms of an 

 arch (fig. 129, f.). Their contraction brings the 

 two adjacent demibranchs together. The smaller 

 bands at the base of the arch (ex.) are the extensor 

 muscles, which cause the demibranchs to stand 

 apart. The action of the two bunds shown in the 



Millimeters 



0.5 



Figure 128. — Terminal groove at the edge of a demi- 

 branch of C. virginica. Longitudinal section of the 

 demibranch. Bouin, hematoxylin-eosin. 



figure is antagonistic. The extensor bands are 

 smaller, probably because the elasticity of the 

 chitinous arches pushes the demibranchs apart and 

 this springlike action means that less force is re- 

 quired of the extensor muscles than of the flexor 

 bands. 



Other muscle bands of the gills, although less 

 conspicuous than the flexors and extensors of the 

 arches are, nevertheless, of great importance in 

 regulating the transport of water through the com- 

 plex gill apparatus and in facilitating the exchange 

 of blood inside the gill filament. Water tubes of 

 the gill can be constricted by the contraction of 

 the muscles underlying the epithelium of the inter- 

 lamellar septa and extending from one lamella to 

 another (fig. 121, il.m.), while the contraction of 

 the transverse muscles of the interlamellar septa 

 compresses the blood vessels. The contraction of 

 the longitudinal muscles of the septa (fig. 121, 

 l.m.) results in the withdrawal and shortening of 

 the entire demibranch. This reaction occurs spon- 

 taneously but can also be induced by stimulation. 

 The contraction of the interfilamental muscles 

 (if.m.) brings together the vertical rods of the gill 

 skeleton, causes the curving of the crossbars, and 

 constricts the blood space of the filament, forcing 

 blood into the pallial veins. 



Contractions afi^ecting only part of the gill cause 

 the blood to oscillate inside the gills. Because of 

 the open nature of the lamellibranch circulatory 

 system the direct return of blood from the gills to 

 the auricles cannot be accomplished by the pump- 

 ing action of the heart. Contractions involving 

 the entire gill apparatus are needed to complete 

 the renewal of blood. 



THE GILLS 



131 



