501 



made to communicate. This communication is rendered most efficient 

 by the shape of the plates on the two sides, which are separated 

 from one another so as to form a large ventral blood-space, &s, which 

 is continuous the length of the gill. This space is continued dorsally 

 between the plates of the two sides, and the spaces of the plates form 

 lateral branches of it. 



Along each side of the ventral portion of each plate, lying just 

 inside, and closely applied to the wall, is a chitinous rod, cr. These 

 rods are much heavier near the median line, become thinner laterally, 

 and taper away at their free ends. The chitinous rod of each side 

 of each plate continues over to form the chitinous rod of the opposing 

 side of the plate adjoining. That is, each chitinous rod is two- 

 pronged, and helps to support two plates. Many of the muscle-fibers 

 of the suspensory membrane, su, are continued along the inner sur- 

 faces of the anterior walls of the plates. These were supposed by 

 Mitsukuri (5) to be chitinous supports, but they were later shown 

 by Kellogg (2) to be muscles. Many of the fibers apparently find 

 insertion on the walls of the plates themselves, but not a few are 

 inserted over the surfaces of the chitinous rods. 



Each gill has two rather large bundles of muscles running its 

 entire length. The one above, ulm, is circular in cross-section and 

 lies near the suspensory membrane. The one below, Urn, is somewhat 

 crescent shaped in cross-section, and lies just below the large blood- 

 space, bs. 



Each plate bears cilia along, and near, its ventral and part of 

 its outer margin, but the cilia are not distributed over its general 

 surface. Near the ends of the chitinous rods the cilia on the edges 

 of the plates disappear, and those between the plates become modified, 

 as described by Kellogg (2), for holding the 

 plates together. The area covered by these cilia 

 is rather limited, and above them the plates 

 are not ciliated. 



The gills, suspended from the body wall by 

 muscular membranes, are sufficiently wide to span 

 the spaces between the mantle and the foot, and, 

 behind the foot, unitedly to span the mantle- 

 chamber itself. The mantle -chamber is thus 

 divided by a movable partition, that ends an- 



Fig. 5. Diagrammatic transverse section of Yoldia limsitula showing the position 

 of the gills in the mantle-chamber. / foot, g gill, m mantle. * shell, su suspensory 

 membrane. 



