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POPULAR SCIENCE REVIEW. 
forming the upper margin of a fold of the lining membranCy 
termed the ‘‘anterior collar ” (Hancock). 
Most of the muscles of the mantle run in a more or less 
longitudinal direction, interlacing in a manner which reminds 
us of the arrangement of muscular fibres which Dr. Petti- 
grew has so clearly demonstrated in the stomach of man and 
other mammals. Some fibres are disposed in a circular manner 
round the two orifices, and, as Van der Hceven says, “ Sphinc- 
teres veluti efficiunt.” (Fig. 1.) They are of the smooth variety. 
A certain space, termed the “pallial chamber ” * or “atrium” f 
(at)', fig. 10), intervenes between the lining of the mantle and 
a sac or bag, yet remaining to be described — 
‘‘Apparet domas intus, et atria longa patescunt.” 
This “ domus intus,” which comprises the gill-sac, the diges- 
tive tube with its accessory glands, and the organs of reproduc- 
tion, lies almost free in the pallial chamber. The gill-sac 
{hr, fig. 10) which has a texture like that of coarsely-woven 
gauze, has, at its upper end, a wide mouth, the margin of which 
is attached to the mantle a little below the “ anterior collar.” 
At its lower extremity is a much smaller aperture, the true 
mouth of the animal. What anatomists would call the “ vis- ^ 
ceral ” portion of the lining membrane is closely adherent to J 
the gill-sac, besides being reflected over the heart and other , | 
viscera, in the manner of a peritoneum. || 
Along that side of the body which is farthest from the aper- 
ture of exit, the two lateral lobes, into which the gill-sac is sup- ' \ 
posed to be resolvable, are separated by certain folds of the '* 
lining membrane which are converted into a longitudinal rod, ' 
termed the “ endostyle.” These folds diverge above, to become i ■ j 
continuous with the lower portion of the anterior collar. On i 
the opposite side of the sac intervenes a similar fold of the lining I 
membrane, the “oral lamina,” which is continuous above, after 
bifurcation, with the anterior collar; and below forms, with the 
lower extremity of the endostyle, the “ posterior cord.” ' ‘ 
Ttie gill-sac is made up of a number of large transverse ^ 
blood-channels, crossed by smaller longitudinal ones, which 
form the margins of narrow elongated windows (“stigmata”) »- 
fringed with cilia, and often having a very beautiful structural 
arrangement (figs. 2 and 4). A nurnbei* of stout longitudinal 
}>ar8 run from one end to the other of the gill-sac, attached 
only where they cross the transverse vessels. At these points 
there is situated a ciliated papilla. The water which traverses 
the sac, after parting with its oxygen to the blood circulating 
• From Lnt. pallium, a mantle. 
t Atrium, the entrance hall of a Foman house. 
ii 
