2 77 
1916-17.] Experiments and Observations on Crustacea. 
portion, which we may in this particular instance call the cephalosome, are 
readily brought into view (to remove the friable spirit-fixed muscles it 
suffices to hold the preparation under the water-tap). Internally the 
cephalosome contains three distinct compartments, the head proper and the 
first and second true thoracic segments. Of these compartments the first 
and the last are large, the interposed somite being very narrow. The separa- 
tion between them is effected by means of calcified septa, which may be 
considered as inturnings of the original free border of some of the somites 
involved. 
The First Intracephalosomic Septum, viz. that between the head proper 
and the perseon, is exceptionally well marked, and therefore of more than 
& 
Fig. 12.- — The first intracephalosomic septum of Glyptonotus. x 2. Semi-diagrammatic. 
a, medial plate-like girder ; b , triangular expansion on which medial 
girder rests ; c , alar plate. 
usual interest. It consists medially of a crescent-shaped vertical plate 
which, resting upon its two ends, appears to support the roof of the cavity 
at this part like a bow-string girder whose upper and lower booms are 
both curved — see fig. 12. Laterally each end of this plate broadens out 
into, or is supported upon, a laminar expansion of triangular shape, which , 
gaining attachment to the floor of the cavity, fills in the angular space 
at the join of tergite and sternite. These triangular expansions are not 
arranged in the coronal plane, the lateral angle of each triangle running 
forwards as well as outwards, the free edge or unattached side of the 
triangle being correspondingly directed posteriorly as well as medially. 
The free edge lies approximately in the same coronal plane as the medial 
girder-like plate, which means that the main body of each triangular 
expansion lies in front of this plane. 
On external examination of the cephalosome the position of this septum 
is distinctly but not obtrusively marked. Previous authors, dealing solely 
