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Webster 1913 Edition

Noun.

[

Cephalo-

thorax

(Zool.)

The anterior portion of any one of the Arachnida and higher Crustacea, consisting of the united head and thorax.

cephalothorax ‎(plural cephalothoraxes or cephalothoraces)

(anatomy) The fused head and thorax of spiders and crustaceans.
- 1874 CE: T. De Witt Talmage, Around the tea-table
  "Come in!" says the gray spider to the house-fly; "I have entertained a great many flies. I have plenty of room, fine meals and a gay life. [...] Philosophers have written volumes about my antennae and cephalothorax."

anatomy: fused head and thorax