A STUDY IN CAECINOLOGY. 39 
liorizontally disposed intersegmental apodemes xiv/xv ; in other words, the 
apodenies separating the last thoracic from the first abdominal segment. 
This sella turcica is not present in all crabs, but where present the arrange- 
ments indicated above are due to the presence of a deep longitudinal mid- 
ventral apodeme in the last one or two thoracic sterna. The effect of this 
deep folding-in of the terminal thoracic sterna is to raise the hinder end of the 
last thoracic sternmn and to throw its arthrophragmal apodemes forward into 
a horizontal position. The edges of the arthrophragms are thus brought into 
contact, and fuse in the middle line. It will be understood from the above 
description that the exhibition of a sella turcica is dependent on the 
formation of a more or less deep apodeme in the mid-ventral line of the 
penultimate thoracic sternum. 
Figs. 13, 14, and 15 are drawings of posterior views of the thorax of the 
lobster, oiRanina dentata, and o'i Dromia vulgaris respectively. In Homanis 
— I should more correctly write Astacus ! — the penuUimate thoracic sternum 
is large and escutcheon-shaped in posterior view : it has a distinct mid- 
ventral apodeme and the central area or inescutcheoii is membranous. The 
terminal sternum consists of two distinct quadrangular pieces united at their 
inner angles, and above these is a curved transverse bar which appears to be 
a special calcification of the membrane connecting the last thoracic and first 
abdominal segment. It is not present in Neplirops norvegicus nor in the 
Crayfish (Potaniohius). On either side of this bar the intersegmental arthro- 
phragms xiv/xv diverge outwards to unite by slender ends with the large and 
peculiarly shaped epimera of the fourteenth segment. From the front edge 
of the sternum just where it passes into the arthrophragm of each side, the 
two narrow and nearly horizontal episternites run forward diverging from 
one another ; only their tips appear in the drawing. There is no sella turcica. 
In Ranina the penultimate thoracic segment, though relatively smaller than 
in the lobster, is similarly somewhat escutcheon-shaped. Both it and the 
ante-penultimate sternum have a deep mid-ventral apodeme, indicated by the 
median suture in fig. 1-i and clearly shown in side view in fig. 9. The last 
thoracic sternum, clearly recognizable by the position of the two strophidia, 
is aliform with tapering wings diverging from one another and passing 
forwards and upwards to unite with the last thoracic epimeron. The arthro- 
phragms xiv/xv are triangular plates arising by their bases from the front 
edges of the V-shaped arms of the last thoracic sternum. Their apices are, 
in the natural position of the animal, directed forwards and downwards ; 
their inner surfaces are concave and their admedian edges meet and unite in 
the middle line, thus forming the sides and floor of the deep trough called 
the sella turcica. Fig. 9 shows the relation of the abdominal nerve ganglia 
and the nerves given off from them to this trough. The anterior ends of the 
arthrophragms in question diverge from one another and, curving outwards 
