SHOET TAILS 51 



or three pairs, only in the Squillidas exceeding that num- 

 ber. The ganglia of the same pair are situated' close to 

 one another, though the commissures may stand a little 

 apart. By the dorsal and lateral extension backwards 

 and generally also forwards of one (or two) of the cephalic 

 segments a shield or carapace is formed covering at least 

 some part of the trunk and sometimes all of it. 



The above characters will suffice for a descriptive defi- 

 nition of the Malacostraca, but it may be proper to remind 

 the reader that the segments are sometimes so intimately 

 coalesced that their separate identity is entirely obscured, 

 and that moreover almost any pair of the appendages, even 

 one so seemingly indispensable as the mandibles, may in 

 certain cases be missing. Absence of eyes is by no means 

 infrequent, and the telson, though perhaps never properly 

 speaking absent, is often, by its close union with the pre- 

 ceding segment, so withdrawn from recognition, that in 

 practice it is spoken of as absent. 



Order 1 . Podophthalma. 



In this order there is normally a pair of compound 

 eyes on movable stalks, the eyes being sometimes absent 

 but never sessile ; the dorsal shield or carapace extends 

 back over the ninth segment or further.. 



Sub-order 1. Brachyura. 



The carapace extends over the whole head and trunk, 

 with occasional exception of the trunk's ultimate and pen- 

 ultimate segments, and is longer than the pleon. In the 

 carapace are excavated orbits and fossettes, hollows respec- 

 tively adapted to receive the stalked eyes and the short 

 first antennas. The third maxillipeds have some of the 

 joints broad and flat, and they form a more or less com- 

 plete operculum to the well-defined mouth cavity. The 

 following pair of appendages are perfectly chelate limbs, 

 commonly called the chelipeds. The next four pairs are 

 adapted for walking or swimming, or rarely may have a 

 prehensile character. In the sternal plastron, or breast- 



E 1 



