ARTIIROPODA. 401 



and post-abdomen (P). Finally, in many arthropods (e.g., the 

 mites or acarina, fig. 403) it is impossible to recognize regions or 

 somites because internal fusion of parts has obliterated the exter- 

 nal evidences of segmentation. 



In order clearly to understand what is meant by head, thorax, 

 etc., requires a consideration of the second character distinguish- 

 ing the arthropods from the annelids, the jointed appendages, 

 which give the name to the former group. The arthropodan 

 appendages are highly developed parapodia, differing in being 

 jointed to the body, in consisting of a series of joints themselves, 

 and in having their intrinsic musculature. As was first pointed 

 out by Savigny, there is but a pair of appendages to a somite, and 

 this belongs to the ventral surface. Hence it follows (Savigny^ 

 law) that if any region shows no external signs of segmentation, but 

 bears more than one pair of appendages, we conclude that the 

 region is a complex of at least as many somites as there are pairs 

 of appendages. Thus the unsegmented head of an insect consists 

 of four somites, the cephalothorax of a lobster of thirteen, for the 

 one bears four, the other thirteen, pairs of appendages. Ontogeny 

 supports this, for in the embryo the somites are clearly visible.* 

 It is not necessary that each somite in the adult should bear ap- 

 pendages, since these may disappear in growth without leaving 

 a trace. 



The appendages subserve many functions (fig. 404). Their 

 primary purpose is locomotion. Locomotor appendages (pereio- 

 poda, feet or legs) are long and consist of a number of well-de- 

 veloped joints which may form flattened oars or may be provided 

 with claws for creeping (8). Besides locomotor appendages there 

 are tactile appendages or antennae (1), chewing appendages (jaws, 

 mandibles, maxillae, 2-4) 9 ^ se f ee * or pleopoda (9) of varying 

 functions, and forms maxillipeds (5-7} transitional between jaws 

 and legs. 



Aside from their tactile function, antennae are characterized 

 by position and innervation. They are always placed in front 

 of the mouth and receive their nerve supply from the supra- 

 03sophageal ganglion, while all other appendages are innervated 

 from the ventral chain. 



The form of the jaws is strikingly modified. One or two 

 basal joints serve for the comminution of food, and these parts 



* This statement is not exactly correct, for in certain insects and in the 

 lobster there is one somite which is entirely lost in the adult. 



