What structural features are characteristic of representatives of the arachnid class?

Common features for the class of arachnids, which distinguish it from other arthropods, is the reduction of the antennae and abdominal limbs. Due to the absence of antennae (antennae), changes can be observed in the nervous system, more precisely, in the brain, where the deutocerebrum (the part of the brain that is responsible for the antennae) is absent. Instead, they were assigned two pairs of mouth limbs on the head, and the chest was endowed with four pairs of walking legs.
It is also worth noting in arachnids such a distinctive feature as the fusion of the head region with the thoracic region (cephalothorax). The presence of pulmonary respiration makes it possible to breathe in the spider-like atmospheric air. Respirations in arachnids are represented by the following organs: trachea or pulmonary sacs, and in spiders, both; the lower representatives of arachnids do not have separate respiratory organs at all.