70 Guide to the Invertehrata. 



GALLERY of the shield, and a row of smaller ones near the front margin. 



Eas?^Side "^^ere are no antennae, but a pair of chelicerse on the front of the 



WaH-case l^ead, followed by two large clawed limbs (maxillary palpi); 



12, Table- behind these are the four pairs of walking-legs. The seven 



^^^^ ' thoracic segments behind the head are broader than the rest of 



the body, and carry upon their under-side the posterior legs, the 



so-called combs, and the generative organs, and four pairs of 



stigmata opening into lung-sacs. The abdomen consists of six 



narrow, elongated segments, without appendages ; the terminal one 



having a recurved poison-spine at its extremity (Fig. 117, t). 



The scorpions are a very ancient tribe of air-breathers, having 

 been met with fossil as far back in time as the Upper Silurian in 

 Scotland, Gothland, and North America ; they are also represented 

 in the Coal-measures of England, Scotland, and Bohemia. (See 

 Table-case 85 and Wall-case 12.) 



IV.—CRUSTACEA. 



This class, represented by the Crab and Lobster and a great 

 variety of other crust-clad animals with jointed limbs, is 

 essentially fitted to inhabit the water, breathing by means of 

 branchiae, or gills. The entire body is usually encased in a hard 

 structure, quite different, however, from the shell of the whelk or 

 the oyster. This defensive envelope protects the softer parts of 

 the body, and also covers the limbs, affording by its overlappings, 

 infoldings, and projections, points of internal attachment for the 

 muscles which move the legs and other organs of the animal. A 

 cursory examination of a lobster, prawn, or shrimp, will show that, 

 like the insects, the body-covering is made up of a number of rings 

 or segments jointed together, to which the feelers, claws, and legs 

 are united by means of movable sockets. To give greater pro- 

 tection to the soft parts of the body, it often occurs that several 

 of these body-rings are soldered together into one piece, as in the 

 crab and lobster; but as all these animals have one pair of jointed 

 limbs to each ring or segment of their bodies, if we find a portion like 

 the lobster's head-shield (called the cephalothorax), which has several 

 pairs of limbs attached to it, we know that this part of the animal 

 is composed of several separate rings or segments united together. 



There are some Crustacea (e.g. Squilla, Talitrus) in which the 

 separate rings can nearly all be seen and counted. In the higher 

 forms, known as the Malacosteaca, there are usually thirteen 



