ore TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
bounded by the usual epidermis formed of a single layer 
of epithelial cells, amongst which sense cells occur 
frequently on the sides. 
Under the epithelium the connective tissue is more 
compact and dense than elsewhere with the series of muscle 
fibres (fig. 45, Br.m.', Br.m.") which have been already 
described. Vascular spaces are of frequent occurrence 1m 
the connective tissue. 
Near the margin to which the gill filaments are 
attached, and running in the same direction as the 
ctenidial axis, are the branchial nerve (fig. 45, V. Br.) the 
afferent branchial vessel and the efferent branchial vessel 
occurring in the order named (ig. 45, Br. affs,) Beaepey 
the nerve being nearest to the body and the efferent 
branchial vessel to the gill filaments. The two blood 
vessels are very close together, and are only separated by 
a narrow bridge of connective tissue. From the two 
corners of the afferent branchial vessel nearest to the gill 
filaments (it is almost rectangular im section) branches are 
given off which pass between the wall of the efferent vessel 
and the surface, and open into expansions on certain of 
the filaments. 
The filaments which make up the lamellae are hollow 
outgrowths from the axis, and arise as simple, straight 
processes, becoming reflected later. Jackson found in 
P. wradians that the young forms examined had 
comparatively a much shorter reflected portion than the 
adults. 
The gills of Pecten are amongst the best examples 
known of the plicate type, that 1s, the filaments, instead 
of being arranged in a flat uniform series, are so placed 
that the lamellae are thrown into a series of vertical folds 
or plicae (fig. 22, and Text-fig. 2). This plication is most 
obvious near the ctenidial axis where the folds are so 
