RESPIRATORY ORGANS IN DECAPODOUS CRUSTACEA. 465 
is near their base that the podobranchs spring. The epipodite 
has not disappeared as it has in the Euphausidse, nor become 
rudimentary as in the Lophogastridse : it has acquired a new 
function, and is of sufficient importance to be preserved. In 
Pengeus it probably has the function of keeping the gills clean, 
but in higher forms (Homarus, Astacus) this office is performed 
by special setse attached to the coxopodite of the appendage, 
and the epipodite has been transformed into a broad lamella 
which serves to separate the gills to some extent and prevent 
their entanglement. In most cases, as in Homarus (fig. 9), the 
podobranch remains quite distinct from this lamella ; but in 
the case of Astacus and some allied forms, all of which inhabit 
fresh water, and are hatched only when fully developed, the 
podobranch exists as a tuft on the epipodite, which at its 
extremity is known as the “ lamina,” and there are branchial 
filaments, exactly similar in structure to those of the tuft, on 
the epipodite itself. This condition is probably due to the 
fusion of the two organs on account of the small compass in the 
egg for the development of each separately. Such fusion is to 
be found in the early life of other Decapods besides the Asta- 
cidae, e.g. Calliaxis (fig. 7) and Calocaris. In the larvae of 
both these forms the podobranch has the appearance of being 
merely a differentiated portion of the epipodite, while in the 
adults the two organs are easily distinguishable from one 
another, although the gill still remains attached to the epipodite 
and does not spring independently from the protopodite, as I 
have been able to verify from a specimen of Calocaris which, 
owing to the kindness of Mr. Pocock, of the British Museum, 
I have been allowed to examine. It appears probable that the 
simultaneous development of the two organs almost on the 
same spot has caused their fusion : in Calliaxis and Calocaris, 
where the larva is free-swimming, separation has soon taken 
place, though the indication of a common origin is maintained; 
in Astacus and its near allies, on the other hand, where develop- 
ment continues in the egg coverings and, consequently, in a 
much limited space, separation takes place to a very small 
extent only. This separation is at the fore end, and the greater 
