296 
Proceedings of the Royal Society of Edinburgh. [Sess. 
to assume that it is here propelled not simply by the vis a tergo of the oral 
appendages, but by some mechanism peculiar to the canal itself — and 
Pearson (1908) has shown that special constrictor and dilator muscles are 
connected with the pharynx of Cancer. The infoldings of the wall of the 
vestibule, which all have a caudal trend, seem to be structural adaptations 
connected with the further propulsion of the food. 
We might perhaps correlate the mobility of the buccal frame with the 
existence of the large chamber of the vestibule. Were the cephalic wall 
rigid on every side, distension of the vestibule with food would necessitate 
Fig. 22. — Glyptonotns. Opening of foregut into midgut as seen from behind. 
Magnified about 5 diameters. 
a, “upper valvular process” of Lloyd ; b, posterior free edge of vesti- 
bule; c, wall of midgut; d, “posterior ampulla” of Lloyd; e, “lower 
valvular process” of Lloyd. 
on each occasion a shifting of an equivalent volume of cephalic contents 
into the thorax — and at this juncture it might be mentioned that in the 
dead animal extreme abduction of the mandibles of itself causes a down- 
ward movement of the buccal frame. Supposing that the ventral endo- 
phragmal skeleton, which is rigidly connected to the buccal frame, were 
ultimately shown to be derived from tergal elements, one might on the 
above correlation hypothesis explain why it has come to lie beneath 
the gut. 
In classifying the Malacostraea the number of oral appendages is of 
fundamental importance. Until we know, however, what maxillipeds are 
for, the statement that decapods have three pairs, members of the Pera- 
