318 CRUSTACEA. MALACOSTRACA 



larval form is the zosea, but a nauplius stage may also 

 occur. 



In many groups of the Malacostraca a dorsal shield or 

 carapace is present, and usually coalesces with the terga 

 of some or all of the thoracic segments, forming a cephalo- 

 thoracic shield (fig. 143, a — c). The telson (e) — a median 

 plate at the end of the abdomen — does not terminate in a 

 caudal fork except in the Leptostraca. Each segment of 

 the body, except the telson, usually carries a pair of 

 appendages. The first pair of antennae (unlike those in 

 the preceding groups) are biramous. In some divisions 

 of the Malacostraca the thoracic appendages are all 

 biramous ; but often, with the exception of some of the 

 anterior appendages, they are uniramous, the exopodites 

 being absent. One or more (often three) of the anterior 

 appendages of the thorax are modified so as to function 

 as jaws, and are known as maxillipedes ; the remainder 

 of the thoracic appendages are used in locomotion. The 

 appendages of the abdomen are biramous; the first five 

 pairs are swimming legs (pleopods) ; the last pair (the 

 uropods, fig. 143,/) are flattened and form with the telson 

 a fan-like tail-fin. In the Malacostraca the position of 

 the genital apertures is constant (p. 291). A pair of 

 compound eyes are usually present. Calcareous ossicles 

 are developed in the stomach forming a ' gastric mill.' 



There are nine Orders of the Malacostraca : — (1) Lepto- 

 straca, (2) Syncarida, (3) Schizopoda, (4) Cumacea, 

 (5) Tanaidacea, (6) Isopoda, (7) Amphipoda, (8) Stomato- 

 poda, (9) Decapoda. The Cumacea and Tanaidacea are 

 not known to occur fossil. 



