1869.J GEOLOGY AND MINERALOGY. 337 



formed of a large plate or buckler (called the Carapace or 

 Cephalothorax), and the rest consisting of ring-like segments. 



The Shell (or Test) of the Lobster well illustrates this. In 

 the Crab, however, the body is more shrunk up, as it were, 

 beneath the Carapace, which is widened and enlarged, whilst the 

 jointed tail-piece is very small and folded neatly underneath. 

 The organs in the Crab are, as it is said, concentrated ; and the 

 traces of the many ring-joints (or " somites ") of which the 

 Crustacean Animal is typically or theoretically constructed are 

 nearly lost to sight. Indeed, if we trace the modifications 

 of structure from one Crustacean to another — from the many- 

 segmented Brine-shrimp to the more definitely jointed Woodlouse 

 and Sandhopper, almost equally ringed throughout the length of 

 their bodies — and through Squills and Shrimps with their carapace 

 in front and their armoured tail behind, and the Anomoura or 

 short-tailed members of the Lobster Tribe, until we get to the 

 Crabs, with scarcely any tail at all, we follow, as it were, the 

 footsteps of Nature in her advance from the lower and simpler 

 structures, with their many times repeated parts and organs, to 

 the higher, more concentrated, more complicated, more specialised, 

 and, in one sense, more perfect type of animal structure. 



We see the carapace flat in the Crab ; in the Lobster it is 

 folded down on either side, and so we have it in many other 

 species ; but this folding is carried a step further in some groups ; 

 the two halves being quite separate at the back, along the central 

 line that is well marked in the Lobster, and becoming the two 

 valves of a two-sided carapace, resembling that of a common 

 Bivalved Mollusc. 



This bivalved structure is not met with among the larger 

 Crustacea, but only in the smaller and frequently microscopic 

 forms. These are members of the group known by the general 

 term "Water-fleas," or Entomostraca, ("shelled insects)." 

 Some live in the sea, some in ponds and rivers. They exist in 

 countless numbers. Like the Sandhoppers, Shrimps, Lobsters, 

 &c., they assist in the health-economy of the watery world ; they 

 are scavengers, using up all dead matters. 



The Crustaceans have been termed " the Insects of the Sea," 

 and well they may, for they not only take the place of Insects, 

 Centipedes, and Spiders in the ocean, on every shore and at 

 nearly every depth, but they emulate the Insect-tribe in the 

 extremes of grace and ugliness. Though they can scarcely be 



