464 
FLORENCE BUCHANAN. 
of Penaeus, which probably had its podobranchs well developed, 
forms like Astacus, Homarus, &c., are to be derived. These 
carry on the ancestral development within the egg, and the 
different gills develop in the ancestral order, the pleurobranchs 
being formed last, and therefore often being not needed by the 
time the creature hatches. In a form like Penaeus, however, 
which has continued to hatch in an early ancestral form, the 
podobranchs which are formed originally at an early stage, and 
before the carapace has grown down to cover them, are wholly 
unprotected, and therefore apt to get more harmed than those 
branchiae which develop later, and are more shielded by the 
carapace. Thus it has come to have been of more advantage 
to the embryo not to develop these outer gills, and natural 
selection has favoured those forms which do not develop them, 
though we still find indications of their having once been 
present in that the rudiments are found in the embryo. 
It would take too long to go into the different branchial 
formulae of all the different groups of Decapods, but I think 
that, taking into account the stretching of the arthro- 
dial mem brane and the time at which it took place, 
the need of protection to the branchiae, the condi- 
tion of the larva when hatched, and probably also 
the condition of the tissues of the creature (some 
tissues requiring more oxygen for the maintenance of the 
individual than others), we can explain all the various 
positions of the branchiae found. One group that I 
might mention particularly is that of the short-tailed Decapods 
or Brachyura. These, as you will remember from the instance 
of the Crab, all have a very much reduced number of branchiae. 
If we look to the development of these forms we find that it is 
very much hurried, and that at the stage in which the gills are 
developed the embryo is so cramped that its thoracic legs 
appear to spring one above the other on the sides of the body 
wall. This would easily account for the suppression and the 
irregularity of the suppression of some of the gills. Turning 
our attention to the epipodites of the Decapods we find that 
these are as a rule present on all the thoracic segments, and it 
