500 



surfaces opposed. The double series of plates is suspended from 

 the body wall by a thin muscular membrane. Anteriorly, the gills 

 pass beneath the overhanging digestive glands, and finally fade away. 

 The extreme anterior ends are continued anteriorly a short distance, 

 in the form of ridges that seem to represent continuations of the 

 suspensory membranes rather than of the gill plates. Posteriorly, the 

 gills likewise become smaller, and are attached, by their ends, to the 

 wall that separates the inhalent and exhalent siphon. Ridges, appar- 

 ently representing posterior continuations of the suspensory membranes, 

 occur ventral to the posterior adductor muscle. 



With the exception of a few of the plates at the extreme anterior 

 ends of the gills, which are sometimes much distorted and swollen, 

 all of the plates are alike in form and structure. Laterally, most of 

 the plates of a gill lie opposite each other. This can be seen by 

 examining a gill from its ventral side. Frequently one or more plates 

 are interpolated on one side or the other, and the plates are made to 

 alternate for a short distance. In a horizontal section, taken through 

 the middle portion of a gill, the opposing edges of the plates are 



alternate. This appearance r 

 I think, is due to the pres- 

 sure and movements of blood 

 in the gill. 



The plates on the two 

 sides of each gill are uni- 

 ted to each other ventrally, 

 Fig. 4, to the suspensory 

 membrane, sw, dorsally, and 

 to the two longitudinal 

 01 muscles, ulm and Urn, that are 

 continuous the length of the 

 gill. The thin walls of each 

 plate enclose a continuous 



'Uro 



Fig. 4. A pair of plates from a gill of Yoldia limatula. bs blood-space, cr chitinous 

 rod. Urn. lower longitudinal muscle, su suspensory membrane, ulm upper longitudinal 

 muscle, v cut surface of a chitinous rod. y cut wall of the gill plate where it bends 

 to join the plate anterior to it. 



space that is traversed only by a few fibers of connective tissue. As 

 the wall of each plate is continuous with the walls of the plates lying 

 immediately anterior and posterior to it (see the unshaded line y, 

 Fig. 4), the spaces of all of the plates of both sides of the gill are 



