271 
1916-17.] Experiments and Observations on Crustacea. 
sternites, like the upper costal cartilages, are arranged transversely, while, 
owing to the forward direction of the lateral parts of sternites 4 and 5, 
corresponding to the upward trend of the inferior costal cartilages, a large 
“angle” is formed not unlike the subcostal angle of the human being. 
This poststernal angle is bridged across by soft cuticle — the circumanal 
cuticle — which is raised into two longitudinal folds, the anal valves , one 
on each side of the anus. The ventral edges of the anal valves are 
rigid, being formed of two chitinified half-hoops, which, hinged at each 
end, open like the jaws of an ordinary metal-mounted leather purse, and 
in the closed position are brought into accurate opposition in the middle 
line — cf. the description by Milne Edwards and Bouvier (1902) of the anus 
of Bathynomus. 
There are at least three structural modifications with which one might 
seek to correlate the increasing tendency, as one proceeds caudalwards, to 
obliteration of the middle parts of the posterior pleonic sternites. These 
are the successive fusions of the corresponding body-rings, the forward 
displacement of structures pertaining to the anus and the development of 
valved uropods. By examination of Glyjptonotus alone one cannot deter- 
mine whether all three are real correlations, still less can one decide 
the relation as regards cause and effect between any two of the four 
modifications involved ; at the same time such problems are of high 
structural interest. 
The anus is situated opposite the dorsal parts of segments 5 and 6, 
and the internal cavity of the pleon may be said to end at the level of 
the sixth segment as in Bathynomus , the narrow space between roof and 
floor of the telson proper being wholly filled with vascular tissue. In 
sagittal section the internal cavity of the pleon is roughly wedge-shaped, 
or the cavity as a whole might be described as slipper-shaped. The roof 
slopes at first gently, and at the fifth segment steeply downwards, while 
the floor inclines uniformly upwards to meet the roof at the narrow sixth 
segment. In this way vertical depth for play of pleopodal muscles is 
retained until just near the termination of the cavity. These muscles in 
solid mass occupy the internal parts of the cavity, leaving in the middle 
a long, narrow, vertical-walled tunnel to be occupied in its dorsal fourth 
by the heart, in its ventral three-fourths by the gut. Only the first pleonic 
tergite presents an inturning of the anterior border, the other tergites 
being flattened calcareous hoops without prominent ventral projections. 
At the same time analogous, vertically arranged, membranous partitions 
subdivide the two lateral parts of the cavity into separate muscular com- 
partments. These are especially marked in the fourth and fifth segments. 
