Figure 120. — Inner cast of the water exhalant system of the gills of 0. edulis. Right- 

 Left — viewed from left side. 



k-icwed from posterior side; 



interlamellar muscles (il.m.) underlie the surface 

 epithelium of the junction between the ascending 

 and descending lamellae. At both ends of the 

 junction near the location of the principal fila- 

 ments the muscles branch off and form well- 

 pronounced bands underlying the chitinous rods. 

 It can be deduced from the pattern of distribution 

 of the interlamellar muscles that their contrac- 

 tions bring the plicae of the two sides of the 

 demibranch together, constrict the blood vessels, 

 and reduce the diameter of the water tubes. 



SKELETON 



A framework of chitinous rods forms a scaffold- 

 ing which supports the entire gill structure. The 

 skeleton can be isolated from the tissues by brief 

 treatment with a weak solution of sodium hy- 

 droxide, which does not affect the chitin. Struc- 

 tural elements cleared by this method are shown 

 in figure 122. The skeleton of each filament 

 resembles a ladder with the horizontal rungs 

 slightly curved on one side and joined to the 

 vertical elements by knobs of chitin. On both 

 sides the supporting ladderlike unit of each 

 filament is joined by cross pieces to tlie next 

 units, forming a continuous framework. The 

 vertical and horizontal bars surround the open- 



ings (ostia) between the filaments (o.). Each 

 skeleton unit supports two adjacent filaments. 

 The vertical bars correspond to the walls of the 

 two adjacent filaments, while the horizontal 

 members (the rungs of the ladder) are embedded 

 in the tissue of the interfilamentar junctions. 

 Two purposes are accomplished by such a pattern. 

 The gill acquires rigidity and at the same time 

 provides strong support for the delicate, sievelike 

 membrane through which water passes into the 

 water tubes. Heavy rods support the principal 

 filaments (fig. 123). At the base of the gills the 

 skeleton rods form massive V-shaped arches 

 embedded in fibrous connective tissue (fig. 124). 



THE FILAMENTS 



The structural unit of the gill is a tubular 

 filament of ciliated epithelium supported by 

 skeleton rods. The central part of tlie filament 

 is occupied by a space which is periodically filled 

 with blood as the gill plates expand and contract. 

 Connective tissue underlies the proximal part of 

 the filament which consists of nonciliated, almost 

 cuboid cells, tightly packed in a two-cell layer. 

 The distal part of the filament is covered with 

 ciliated cells (fig. 125). Bulky nmcous cells 

 occur at irregular intervals and discharge their 



126 



FISH AND WILDLIFE SERVICE 



