NOTES ON A TRACT OF MODIFIED ECTODERM. 97 
which it is in close connection. On removing the shell 
the body is seen to be composed of two distinct portions. 
(a) The pallial chamber, which occupies the larger and 
anterior part of the body, communicates freely with 
the exterior, and contains the arms. This chamber is 
separated posteriorly from the visceral cavity by a firm 
body wail which is continuous with the dorsal and ventral 
lobes of the mantle; when the shells are closed the 
margins of the mantle touch, and the pallial chamber 
must be shut off from the exterior. 
(b) The visceral cavity. Posterior to the pallial chamber 
there is a relatively small chamber, in which are placed 
(1) the digestive organs, z.e. the alimentary canal and a 
large glandular organ communicating with it; this is 
suspended in the cavity and connected with the body-wall 
by a membrane which may be termed the mesentery ; 
(2) portions of the reproductive organs; (8) the muscles 
which move the shell. 
The Arms form a very prominent feature of the pallial 
chamber (PI. TV., figs. 2, 3 and 4), and call for some descrip- 
tion. They are attached to and in structural connection 
with the anterior body wall from the median line to some 
distance on the right and left (see Pl. IV., fig. 4); from 
their base they are continued laterally and posteriorly, 
then make one large coil anteriorly and medianly, followed 
by several smaller coils in the same plane. From base to 
apex the arms are provided with a fold and a double row 
of alternating tentacles (Pl. II., figs. 2, 8 and 4); the 
fold is always dorsal to the tentacles. The base of the 
arms is pierced by the cesophagus. In the arms there are 
three sets of canals (see Lingula, Pl. III1., fig. 2; Crana, 
Pim fies3). 
(a) A canal containing the brachial muscle, which com- 
municates with the tentacular canals. 
7—2 
