228 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



found in America and in the Moluccas. Tliey are usually 

 classed as a distinct order of the Crustacea^ termed Xipho- 

 sura or Pcecilojyoda. 



The body of Lhnulus (Fig. 58) is naturally divided into 

 three parts, which are movably articulated together. The 

 most anterior is a shield-shaped portion, curiously similar in 

 form to the head of a Trilobite. Its convex dorsal surface is 

 similarly divided into a median and two lateral regions; its 

 edges are thickened, and its posterior and external angles are 

 produced backward. At the anterior end of the median re- 

 gion two simple eyes are situated, and at its sides are two 

 large compound eyes. The sternal surface presents, ante- 

 riorly, a flattened suhfrontal area, behind which it is deeply 

 excavated, so that the labrum and the appendages are hidden 

 in a deep cavity formed by its shelving walls. The middle 

 division of the body of Limulus exhibits markings which in- 

 dicate that it is composed of, at fewest, six coalesced somites; 

 its margins are spinose, and its excavated sternal face lodges 

 the appendages of this region. 



Fig. 58.—^, Limulus inoluccanus{AoT^K\ view). 5, L. rotund'icauda (ventral view) 

 (after Milne-Edwards): a. anterior; b, middle division of the body ; c, telson, d; 

 subfrontal area; e, aiitennules ; f^ autenuae ; g^ operculum; A, branchiferoas ap- 

 pendages. 



The terminal division is a long, pointed, and laterally ser- 

 rated spine, which is termed the telson. 



