26 
Guide to Crustacea. 
Table-case primitive characters. In the large number of the somites and the 
N°. 1 . uniformity of the limbs, as well as in some points of internal 
structure (heart, nervous system) they approach more closely 
than any other living Crustacea to the hypothetical ancestral type 
of the Class. In some respects, however, such as the reduction of 
the mouth-parts, they are considerably specialized. 
The order includes three Sub-orders (sometimes ranked as 
Orders) the members of which differ widely in external appear- 
ance. They are found in fresh water or in brine pools. 
Fig. 7. 
Apus cancriformis, from Kirkcudbrightshire, slightly enlarged. 
[Table-case No. 1.] 
In the Sub-order Anostraca there is no carapace and the 
animals have a more worm-like appearance than is usual in 
Crustacea. The eyes are set on movable stalks. The males are dis- 
tinguished by the remarkable development of the antennae, which 
form complicated clasping organs for seizing the females. This is well 
shown in the specimen of Streptocephalus rubricaudatus exhibited. 
In the Sub-order Notostraca the carapace forms a broad 
dorsal shield, resembling, at first sight, that of the Arachnidan 
King-crabs. Apus cancriformis (Fig. 7) is found in fresh-water 
