INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 2 - H.I. Shcherbak - 1996

PHYLUM ARTHROPODA (ARTHROPODA)

SUBPHYLUM CHELICERATA (CHELICERATA)

Chelicerates are a highly diverse group of Arthropods that have successfully colonized both terrestrial and aquatic habitats. The vast majority of modern chelicerates are land-dwelling, with only about 5% of species inhabiting marine and freshwater environments.

While the body plan of various chelicerate groups varies significantly, a set of common features is shared without exception. Specifically, the body of chelicerates is divided into two tagmata: the cephalothorax (prosoma) and the abdomen (opisthosoma). The cephalothorax bears six pairs of appendages. Antennae and antennules are entirely absent in chelicerates. The first pair of appendages, situated in front of the Mouth, often terminates in a pincer and is called the chelicerae (hence the name of the subphylum). Behind the mouth lies the second pair of appendages, the pedipalps (palps), which exhibit diverse structures and Functions. The remaining four pairs typically function as walking legs. In most chelicerates, abdominal appendages are lacking; when present, they are highly modified to perform respiratory, reproductive, or other specialized functions.

Unlike other arthropods, the Brain of chelicerates consists of the protocerebrum and tritocerebrum; the deutocerebrum, which innervates the antennae (as seen in insects, for example), is absent.

The Subphylum Chelicerata comprises A number of distinctly distinct animal groups categorized by various zoologists into taxa of different ranks. In this work, we recognize two classes within the subphylum: Merostomata and Arachnida.