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1916-17.] Experiments and Observations on Crustacea. 
extremities (the pear stalks) at some little distance above the floor — cf. 
fig. 16. In this way a medial ventral channel is left for the nerve-chain, 
the foregut being supported above these structures — see also fig. 21. 
The anterior cheek or surface of each of these alar jpiriform bodies 
differs from its dorsal and posterior surface (which is wholly chitinous) in 
being rigid and calcareous. The anterior of each pear is occupied with 
soft tissue, which seems to be a cylindrical or tubular structure folded two 
or three times upon itself. This internal tissue, which is not fatty, as Lloyd 
reports in the case of Bathynomus, is most probably a part of the maxillary 
excretory gland. 
The calcareous anterior surface, which we may call the alar plate , is 
arranged in a vertical transverse plane; it springs from the alar bar of 
the maxillo-sternal framework. On the posterior aspect each alar plate is 
slightly concave to accommodate the internal soft tissue of the pear. The 
chitinous surface, or alar capsule , convex externally, concave internally, is 
like a skull-cap ; attached along the upper border of the alar plate, it is 
folded backwards and downwards from this line of attachment so as com- 
pletely to cover the underlying tissue; at its inferior edge it seems to be 
free, and it is readily lifted from behind. With a little care it may be 
removed almost complete. It is thin, translucent, and flexible, and appears 
to act simply as a covering to the underlying soft tissue. 
In contrast with the attached capsule the alar plate is a complicated 
structure. Dissected free of the softer parts it presents an appearance 
that Lloyd has compared to the wings of a butterfly — see fig. 12. From 
the central body of the plate spring four pterygoid processes , one at each 
corner, i.e. a superior pair and an inferior pair. For descriptive purposes 
each alar plate might be considered as held in position by four attachments 
to surrounding parts. Two of these attachments occur at the extremity 
of pterygoid processes ; the other two attachments are brought about by 
means of special calcareous bars situated in front of the plate, one of these 
bars being a member of the pair of rods already mentioned as springing 
from the dorsal aspect of the keel of the buccal frame. 
The superior lateral and the inferior medial pterygoid process end 
freely. The superior medial process is tied to its neighbour of the opposite 
side by a band of soft tissue. The inferior lateral process is fused to the 
transverse bar in the buccal frame which separates the articular foramen 
for the maxilliped from the articular foramina for the two maxillae, 
whence it follows that the alar plate can move only with movements of 
the buccal frame. 
When viewed from behind the alar plate seems to be rigidly fixed only 
