600 K, MITSUKURI. 
work consists of a trough (seen in cross-section at s, fig. 5; 
longitudinally from below in fig. 8; diagramatically repre- 
sented in fig. 7), which runs along the whole length of the 
gill, and from which a pair of closely-applied parallel 
branches (A, figs. 5, 6, 7, and 8) is given off into each plate. 
The trough is divided into two unequal parts: an upper 
larger and a lower smaller, by a cross piece (c. p., figs. 5 
and 7), which stretches from one side of it to the other, a 
little below the middle. This cross piece is not, however, 
continuous, but is pierced through by oral openings (0 2, figs. 
7 and 8) whenever branches are given off laterally to the 
plates. The space enclosed between each pair of closely- 
applied branches (see A, figs. 6, 7, and 8) is connected with 
the lower compartment of the trough by means of somewhat 
circular openings (0p, and o’ py’, figs. 7 and 8) found near 
the bottom. In fig. 8 the letters a,a,a,are placed opposite 
each pair of the branches that go intoa plate. It will be 
seen how one half of the chitinous support of one plate, after 
forming an arch at the trough, turns round to enter the 
next plate in succession, and to constitute there one half of 
the support of that plate. The framewerk treated with pot- 
ash, and sometimes without any treatment, shows marked 
longitudinal striation (fig. 8), and some of its fibres stick- 
ing out at the broken edge beyond the others resemble in 
appearance the fibres found in the suspending membrane, 
at ¢and uw, fig. 5, and give reasons for thinking that the 
whole chitinous framework is nothing but the fibrous tissue 
found in other parts cemented closely together and forming 
one cohering mass. 
Although, owing to the state of the specimens, I have 
obtained only here and there evidences of cilia, it seems 
reasonable to suppose that the whole gill is covered with 
cilia. On two rows of cells (0. f., fig. 6; d. a., fig. 5) on the 
lower edge of the plate I believe there are larger cilia than 
on the rest, as I have now and then seen their remains, and 
as, without any question, cells in the corresponding posi- 
tions in Yoldia have long and conspicuous cilia. 
Yoldia limatula, Say. 
Yoldia resembles Nucula in several structural peculiari- 
ties—in its well-developed labial palpi, with their peculiar 
food-procuring appendage, in its feather-like gills, in the pos- 
terior position and comparatively small size of the gills, and 
the consequent absence of the division of the mantle cavity 
into the supra and infrabranchial chambers. It differs from 
