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1916-17.] Experiments and Observations on Crustacea. 
Lloyd (1908) has given the name of “sternal alee,” will be more minutely 
described under the heading of Ventral Endophragmal Skeleton. 
The Second Intracephalosomic Septum. — The shape of this septum, in 
end elevation, is shown in fig. 14. More feebly developed than the first, 
with which it fuses laterally, it exactly follows the curved line previously 
referred to as marking the separation between cephalosome proper and 
second true thoracic somite, which means that its lateral portions lie 
considerably anterior to its middle part. The free edge of each lateral 
triangular expansion is the chief feature that renders the identity of this 
part of the septum distinct in the interior, for the main body of each 
expansion appears to fuse with the corresponding, much larger, triangular 
This septum intervenes between the first and second true 
thoracic somites — cf. fig. 16 . 
expansion of the first septum. As we shall see later, there is almost 
complete separation between the two by means of a narrow cleft which 
leads in from the exterior on the lateral part of the cephalosome. 
A Function of the Intracephalosomic Septa. — Whatever the conditions 
that led to the formation of the complex internal skeleton described above, 
one result certainly achieved is that the head region of the animal is 
strongly braced against compression applied from without. In seas where 
glacial boulders are dropped from above, and where even deep-water 
animals are tossed about by great surges, liability to accident by crushing 
is by no means remote. Especially vital parts of an isopod are the head 
and the abdomen. In Glyptonotus the rigidity of the latter largely 
depends on the strong mid-dorsal ridge, that of the former on the system 
of internal struts. In other isopods there are rudimentary homologues of 
the highly developed first intracephalosomic septum found in Glyptonotus. 
These take the form of two inturnings of the dorso-lateral portion of the 
posterior border of the cephalon, and are to be seen in Ligia; also in 
