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GAMMARUS are members of the Malacostraca
Class Malacostraca
The Malacostraca include about two thirds of all crustacean species, and
contains all the larger forms, such as woodlice, shrimps, lobsters and crabs .
Their body comprises:
head (6 segments) with a pair of antennules (A1) and antennae (A2) and
mouthparts, thorax (8 segments and abdomen (6 segments). All thoracic segments
and most abdominal segments have a pair of appendages, which are primitively
biramous. Gills occur on or close to the bases of the thoracic appendages.
The three main orders of malacostracans are the ISOPODA
(which includes Asellus the water louse) , the AMPHIPODA
(which includes the Freshwater 'shrimps' Gammarus spp.) and the Decapoda which
include the marine shrimps and crabs.
The DECAPODA are the largest group of
crustaceans, numbering some 8,500 known species. Most are marine, but there are
a few fresh water species, the crayfish (Astacus sp.) being a well known
example. There are also some terrestrial crabs.
Body structure
Crustaceans are primitively highly segmented animals, each segment bearing a
pair of limbs. More advanced crustaceans, however, have fewer limbs which
have tended to become specialised.
The crustacean head is fairly uniform, having five pairs of appendages
comprising two pairs of antennae, a pair of mandibles and two pairs of maxillae.
The thorax consists of a series of limb bearing segments, some of which may
have become fused with the head so that their appendages have become
incorporated into the mouthparts. The abdomen may or may not have appendages.
The appendages are typically biramous, that is having two branches, but may
become secondarily uniramous* . As in other arthropods, there has been a
tendency towards fewer appendages and more specialised function.
Crustaceans, unlike most other arthropods, have a calcified cuticle.
Prior to moulting the calcium is reabsorbed and then deposited in the new
cuticle. This is probably due to the inability of crustaceans to absorb large
amounts of calcium rapidly from their surroundings in a relatively short period
of time.
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