prosoma

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The anterior of the two or three main divisions of the body of certain invertebrates, especially the cephalothorax of an arachnid.

from The Century Dictionary.

• noun The anterior or cephalic section of the body of a cephalopod, bearing the rays or arms; the head or anterior part of any mollusk, in advance of the mesosoma.
• noun In dimyarian lamellibranchs, a region of the body in which is the anterior adductor muscle, and which is situated in front of the mouth: it is succeeded by the mesosoma.
• noun In Cirripedia, the wide part of the body, preceding the thoracic segments: in the barnacle, for example, that part which is situated immediately behind the point of attachment of the body to the shell on the rostral side. Darwin. See cuts under Balanus and Lepadidæ.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) The anterior of the body of an animal, as of a cephalopod; the thorax of an arthropod.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology The front segment of a body which is divided into two or more segments, especially the cephalothorax of an arachnid.