•They are distinguished from other groups of arthropods for :

• possession of two-parted limbs.• the nauplius form of the larvae.

•Their fossile apparently unchanged since the Triassic period

Scientific studies =CARCINOLOGY

•Crustaceans is a very large group ok arthropods, which includes: crabs, crayfish, krill and barnacles

Their body is divided into three region :•Head •Thorax•Abdomen

In the head we find also the chefalothorax .

The crustacean’s body is entierly covered by an hard exoskeletron.

Each somite, or body segment can bear a pair of appendages:

HEADtwo pairs of antennae

the mandibles

maxillae

TORAX

legs

To walk To feed

Other carachteristicOther carachteristic:• Appendages are tipicall biramous• The main body cavity is an open circulatory system, where blood is pumped into

the haemocoel by a heart located near the dorsum. • The alimentary canal consists of a straight tube for grinding food and a pair of

digestive glands that absorb food; this structure goes in a spiral format.• The majority of crustaceans are aquatic, living in either marine or fresh

water environments

•Arachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata.

•They comprise over 100,000 named species,including spiders, scorpions, ,ticks, mites and Solifugae.

•Basic characteristics of arachnids include four pairs of legs and a body divided into two tagmata: the cephalothorax and the abdomen.

•Eight legs•Two pairs of appendages• Their body is organized into two tagmata

cheliceraepedipalps Prosoma

Abdomen

Head

Thorax

Preabdomen

Postabdomen

•The excretory glands of arachnids include four pairs of coxal glands along the side of the prosoma, and one or two pairs of Malpighian tubules;•The blood of arachnids is variable in composition, depending on the mode of respiration;•The heart is located in the forward part of the abdomen.