ELEDONE. 525 
short time, when there is no apparent cause for 
disturbance. 
(II) Respiratory MercHaAnism. 
1. Gills—Eledone, in common with all the living 
Cephalopods except Nautilus, is Dibranchiate, 1.e., it has 
a single pair of gills. Morphologically these gills 
represent part of the inner surface of the mantle, which 
has been specialised for respiratory purposes. They are 
situated in the mantle cavity, laterally to the visceral 
mass. When the mantle is cut open, and turned back (as 
in fig. 11), they may be seen slanting obliquely outwards 
and upwards from the posterior end of the visceral mass. 
In fig. 8, which shows the left division of the mantle 
cavity, only one may be seen. ‘They are attached both to 
the mantle and to the visceral mass. The gill may be 
considered as a slender hollow cone, with the apex 
pointing upwards and outwards. The hollow which forms 
the base leads into the cavity of the cone, whose walls are 
formed by the branchial leaflets. ‘The cone is, however, 
laterally compressed, and is attached to the mantle 
dorsally, but free ventrally. Along this dorsal attached 
axis of the gill runs the afferent vessel, while along the 
free one runs the efferent vessel, so that the plane joining 
these two axes bisects the gill, and is at right angles to 
the plane of the mantle. The tip of the cone 1s situated 
just posterior to the level of the anus, while the base is 
slightly behind the urinary papilla. Below this may be 
seen the branchial heart, which, receiving blood from the 
body, pumps it on through the afferent vessel to the gill 
for aeration. Posteriorly the gill is bound to the visceral 
mass by—(1) The two muscular bands, mentioned in the 
description of the mantle cavity. The first runs down the 
ventral side of the efferent vessel, from its tip, and then, 
