1916-17.] Experiments and Observations on Crustacea. 269 
complicating factor, involving a study in itself. Even in Glyptonotus one 
cannot be certain whether distension or retraction of the articular mem- 
branes in the limbs does not accompany body flexion. This could be 
settled by taking plethysmographic records, perhaps even of a single limb, 
during flexion or extension of the perseon ; similarly a manometer would 
record any pressure changes within the animal. What I wish to bring 
out is that design and correlation of parts are susceptible of experimental 
treatment, and until the subject comes to be systematically handled in this 
way we can hope to have but the vaguest ideas on the matter. 
The position of the centres of rotation in the hinge between two somites 
is determined not so much by the conformation of the hard structures as 
by the presence of very short stout ligaments at the essential points, viz. 
at the level of the junction of the epimera with the tergites. It would 
serve no immediate purpose to describe the structure of the bearing 
surfaces between the somites, more especially as the epimeral bearing 
surfaces vary in form along the series. 
The Pleon and its Appendages. 
It is only in the rarest cases among Isopoda that one finds the telson 
distinct from the sixth segment of the pleon. In Glyptonotus the seventh, 
sixth, fifth, and fourth primitive segments are all fused together, the 
separate fusions having obviously occurred in succession from the caudal 
end forwards. Only three of the six interpleonic articulations permit of 
movement. 
The fusion between segments 4 and 5, obviously the most recent, and 
overlooked by previous authors, is present in both species of Glyptonotus. 
The line of junction between segments 5 and 6 is sharply marked on the 
medial dorsal ridge, where the anterior of the two overhangs the posterior ; 
less sharply, yet in parts with sufficient distinctness, between the lateral 
border and the medial ridge. Caudal to the fifth segment the medial ridge, 
lying at a new and lower level, is not interrupted by any cleft, yet the 
original line of junction between segments 6 and 7 has not been entirely 
obliterated. On its dorsal aspect the telson proper is homogeneously 
pitted with fine depressions, whose distribution corresponds with the area 
ventrally covered by soft cuticle ; this pitted region is likewise, and as a 
consequence, the chromatophore-bearing region. In tracing the anterior 
limit of these pits, in some specimens mapped out with quite a sharp 
anterior boundary, one traces the original line of separation between the 
sixth segment and the telson, the general situation of the line being further 
rendered plain by a pair of short transverse depressions, one immediately 
