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1916-17.] Experiments and Observations on Crustacea. 
2. The Legs. — The peculiar articulation, analogous to a spheroidal bony 
joint, between the coxopodites and basipodites of the thoracic limbs, is 
described in detail. A comparison is instituted between the full flexion- 
complex of the isopodan and of the brachyuran walking limb. The 
peraeopods are contrasted with the gnathopods of Glyptonotus, and 
functional peculiarities pertaining to each group of limbs are discussed. 
3. The Perceon. — The medial split in the thoracic sternites is interpreted 
as a device for allowing of distension of the body, say, after a meal. 
The arched thoracic somites articulate with each other in a special way, 
not by a crossed articulation, like that in a pair of scissors, but by a rocker 
articulation, like that in a spring clothes-pin ; this combination of arch and 
rocker articulation is interpreted as a means of preventing change of internal 
volume in body flexion. The phenomena pertaining to the moulting process 
throw light upon the numerical grouping of the successive fusions of coxae 
with somites which have occurred in isopods. 
4. The Pleon. — The pleon consists of four movable portions, the last 
four of the seven constituent pieces being welded together. Of the four 
fused segments, only the first has a complete sternite, like those of somites 
1, 2, and 3. In the skeleton of the pleonic floor a wide opening is thus left 
posteriorly, which is closed by a diaphragm of soft cuticle ; part of this 
diaphragm is elevated into two long parallel folds or valves, one on each 
side of the anus. 
5. The Pleonic Appendages. — The protopodite of each of the anterior 
three pleopods is composed of two complete pieces ; a third piece more 
proximally situated is interpreted, not as evidence of an additional primitive 
segment, but as a secondary development in the articular membrane. The 
uropods have acquired their present position by a forward rotation of the 
sternite of the sixth primitive somite, and the gill-directed surface of the 
uropod corresponds to the posterior surface of the pleopods. It follows 
that what has commonly been called the exopodite of the uropod is a real 
exopodite. In addition to the pleopods, the internal walls of the whole 
uropodal chamber subserve a respiratory function. 
6. The Cephalosome. — Two thoracic somites are fused with the head to 
form a cephalosome. By welded inturnings of the anterior border of these 
two and of the posterior border of the mandibular cephalic somite a strong 
internal bracing is formed. The tergites of the maxillary somites have 
apparently disappeared from the dorsal aspect of the cephalon ; the endo- 
skeletal structures described by Lloyd in Bathynomus, and by him named 
“ sternal alae,” functionally correspond to these tergites in so far as they 
serve for attachment of muscles for the maxillae. These “ sternal alae ” 
