460 
FLORENCE BUCHANAN. 
being formed close together in the larval form of one of the 
Decapods, Calliaxis (fig. 7), where we see two of the branchiae 
(b and c) being formed almost from the same spot. How far 
they are apart from each other depended in all probability 
originally upon the exact time at which the special branch 
developed in relation to the stretching of the membrane cover- 
ing the joint of the limb. When their position had become 
definitely established they finally, in most cases, but yet not 
quite in all, as we see from the Calliaxis larva, developed 
straightway in their respective places even when developing 
at the same time. Calliaxis and the forms allied to it (i. e. all 
the Thalassinidte) never have gills on the epimeral wall, 
though they have very well-developed ones on the arthrodial 
membrane ( b and c) as well as on the epipodites (a). This as 
well as the structure of the gills seems to poiut to a more 
ancestral condition than that of most other Decapods. 
The advantage in the branchise being situated immediately 
on the bases of the appendages has ceased to exist, as it had 
also in the higher Schizopods, for the branchiae have become 
more complicated (in comparison with the simple ones of the 
Euphausidse), and require protection from surrounding ob- 
jects, though, of course, still requiring continuously to be 
bathed with fresh supplies of water. Already in the Lopho- 
gastridse we find three branches of the compound gill covered 
in by the carapace (fig. 10), while the fourth is bent over on 
the ventral surface, and is thus also to some degree protected 
from being hurt by anything with which the creature comes in 
contact. In the Decapods we find all the gills, even those 
attached to the basal joint of the limb, protected by the cara- 
pace (fig. 11), and so closely closed in by it in most cases as to 
lie in a special branchial chamber through which water is 
driven, as in the Crayfish, by the continuous movement of 
the scapliognathite or exopodite of the second maxilla. This 
covering in of the gills for protection may have been accom- 
panied by a tendency to vary in the position assumed, and 
when once varied natural selection may have favoured the 
variation. 
