But it is also of interest to ascertain what is the use of the two
additional arms in the Decapoda. They differ from the other arms in being
much longer, and provided with sockets into which they can be retracted,
and suckers on them are limited to the terminal region. In the majority of
zoological books in which Cephalopoda are described, nothing is said of
the use or function of these two special arms. Observation of the living
animal in aquaria has shown that their functions is to capture active prey
such as prawns. They act as a kind of double lasso. Sepia, for instance,
approaches gently and cautiously till it is within striking distance of a
prawn, then the two long tentacles are suddenly and swiftly shot out from
their sockets and the prawn is caught between the suckers at the ends of
them. Another example is afforded by the masked crab (_Corystes
cassivelaunus_). This species has unusually long and hairy antennae. These
are usually tactile organs, but it has been found that the habit of
_Corystes_ is to bury itself deep in the sand with only the tips of the
antennae at the surface, and the two are placed close together so as to
form a tube, down which a current of water, produced by movements of
certain appendages, passes to the gill chamber and provides for the
respiration of the crab while it is buried, to a depth of two or three
inches. The results of the investigation of habits and functions may be
called Bionomics. It may be aided by scientific institutions specially
designed to supplement mere observation in the field, such as menageries,
aquaria, vivaria, marine laboratories, the objects of which are to bring
the living organism under closer and more accurate observation. The
differences between the methods and results of these two branches of
Biology may be illustrated by comparing a British Museum Catalogue with
one of Darwin's studies, such as the 'Fertilisation of Orchids' or
'Earthworms.'

Other speculations in Biology are related to Taxonomics or Bionomics