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Proceedings of the Royal Society of Edinburgh. [Sess. 
The Hindgut. 
Even when it contains no food the hindgut remains widely open in the 
preserved specimens. In its thoracic part it is shaped like the end of a 
spindle ; in its pleonic part its cross-section is quadrilateral. The interior 
of the thoracic portion is perfectly smooth ; that of the pleonic portion is 
thrown into innumerable slight ridges, which confer upon it a reticulate 
appearance. These ridges, which tend on the whole to run transversely, 
probably correspond to strands of muscular tissue under the lining, for 
Miller (1910) has shown that such bundles of muscular fibres occur in the 
corresponding ridges in the posterior part of the hindgut of the crayfish 
( Cambarus ), in which animal the anterior part of the hindgut is likewise 
smooth. Round the anus the ridges have a radial arrangement — cf. Miller’s 
account of radiating muscular bundles round the anus of the crayfish. 
The anus is smaller than one might expect from the size of the anal 
valves. The arrangement of parts in this region may be described as 
follows. The circumanal cuticle forms a strong diaphragm, which closes 
the wide posterior opening in the ventral skeleton of the pleon. Though 
elevated at one portion into two deep parallel folds (the anal valves), the 
cuticle still extends across the floor of the space between these valves. At 
the very posterior part of this floor the cuticle is perforated by the anus, 
and to the edge of the perforation, but to no other part of the diaphragm, 
the hindgut is attached. 
It is possible that the pleopodal muscles may play some part in the on- 
ward propulsion of food material along the pleonic portion of the hindgut. 
When working with Gammaras — see Tait (1908, 1910) — I had frequent 
opportunity of observing that this animal, replaced in sea-water after a 
longish sojourn in moist air, invariably begins to defsecate. The observa- 
tion was of interest as pointing to the existence of a natural provision for 
maintaining the hygiene of the beach, waste being eliminated only when 
the tide is up. The process may be an independent reflex, in which con- 
tact of some part ( e.g . the anus) with sea-water acts as the exciting 
stimulus, but it may also, in part at least, be a secondary result of the 
renewed activity of the pleopods consequent upon immersion. The same 
phenomenon is observable in the case of Ligia, whose pleopods also become 
active on immersion. It is likewise to be seen in Carcinus , whose gut 
is not sandwiched between pleopodal muscles ; which fact rather suggests 
that the process is an independent reflex. It is worth while mentioning, 
however, that Carcinus helps to extract faecal masses from the anus 
by means of its chelae, whence one might infer that the local mechanism 
