2 68 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



the scale of Articulata, to make the infinite variety of 

 forms constituting this department of animals. 



Our object, then, is to show that the skeleton of an 

 arthropod consists of naught else than a series of similar 

 segments, modified according to the place and function 

 in the series. 



Take any articulate animal, say an arthropod, like an 

 insect or crustacean. Make a cross section and view it on 

 end (Fig. 169). We see an external bony ring inclosing 

 everything, and attached to it a pair of jointed append- 

 ages, right and left. Now, the whole skeleton is made up 

 of such rings and appendages, repeated and modified. The 

 repetition of the rings gives rise to a hollow, many-jointed 

 cylinder or barrel, and the repetition of the appendages 

 to a continuous row of such on each side. Each ring, 

 with its pair of appendages, is called a somite, or body 

 segment. Such is the simple idea or archetype of an ar- 

 ticulate animal ; but, in fact, these ideal rings and append- 

 ages are very variously modified for function. Some 

 are modified for swimming appendages, some for walk- 

 ing appendages or limbs, some as food-gathering append- 

 ages (jawfeet), some as biting appendages (jaws), some 

 as sense appendages (eyes, ears, feelers), but all made 

 on the same plan, only modified. The manner of modifi- 

 cation is sometimes by enlargement, sometimes by dimi- 

 nution and even disappearance, sometimes by coalescence 

 and consolidation of several into one piece. 



For example, take one from about the middle of the 

 scale — say, a crawfish or a lobster. This animal consists 

 of about twenty-one rings and pairs of appendages (Fig. 

 170). In the tail region or abdomen the seven rings 

 are all separate and but little modified, but their jointed 

 appendages are greatly modified for swimming, £>' ; 

 those of the last joint but one are enlarged and flat- 

 tened, and, together with the flattened last joint, which 



