466 
FLORENCE BUCHANAN. 
parts of the two organs remain fused throughout life, giving the 
appearance of an epipodite forming gill filaments. In some cases 
(Astacoides) separation never takes place at all. In Homarus, 
although there is the same tendency of the two organs to be 
formed together, as shown by the attachment of the podo- 
branch to the base of the epipodite, yet, as the larva has 
become free-swimming before differentiation takes place, 
separation takes place at the same time, and the podobranch 
never is fused, except just at its base, to the epipodite. 
To refer now briefly to the other groups of the Malacostraca, 
which I have until now put aside in considering the histori- 
cally most interesting group of the Decapods, we come first to 
the Stomapods, of which Squilla is an example. These prob- 
ably are to be derived (see classif.) from a Malacostracan form, 
whose swimming and respiratory organs were not yet fixed to 
the thoracic region as they are in the Archischizopod. We 
know that the Stomapod does not undergo the same changes 
in development as the forms with which we have hitherto been 
dealing, but that, instead of the midbody being developed last 
as it is in the Schizopod and Decapod, this becomes developed 
before the hind body. The thoracic appendages therefore 
develop early, and probably before any special respiratory 
apparatus begins to be needed, while the abdomen is only 
afterwards developed, and its appendages become the chief and 
most active swimming oi’gans. The swimming function, there- 
fore, which in the Archischizopod is the part of the thoracic 
limbs, is here undertaken by the abdominal appendages ; and, 
as in the Schizopod, the gills have developed behind the 
thoracic swimming appendages, so in the Stomapod they 
have developed behind the abdominal swimming appendages 
and are present as branchial tufts attached to the exopodite, 
not in any way representing an epipodite. It is worthy of 
note that it is not only the respiratory organs but also the 
heart, generative organs, &c., in the Stomapods that develop 
in the abdominal instead of in the thoracic region. This 
probably has also to do with the reversion in the development 
of the two regions. 
