210 



THE RISE OF ANIMAL LIFE 



The primitive appendage, one which ap- 

 pears in the early embryology of all Crus- 

 tacea, is said to be biramous, because it 

 branches into two parts. It persists in its 

 primitive condition in the swimmerets, lo- 

 cated on the underside of the abdomen. 

 The single basal portion, the protopod, is 

 attached to the body and two branches ex- 

 tend from it, the endopod, toward the 

 median line and the exopod away from it. 

 The original function of such an appendage 



exopod has been greatly reduced or lost 

 entirely. 



Structures that have a common origin, 

 such as in the case of these appendages, are 

 said to be homologous, and when they are 

 on the same animal they are said to be 

 serially homologous. This introduces the 

 principle of homology which is illustrated 

 throughout the animal kingdom and is very 

 important in determining animal relation- 

 ships. Homologous structures have a com- 



sensory 



cheiuing 



breathing 



carrying eggs 

 copulation 



Fig. 11-10. Young lobster, showing the undifferentiated appendages, together with the ap- 

 pendages as they will appear in the adult animal. Note how they are modified for specific 

 functions. In the young form the appendages are designed primarily for locomotion, in the 

 adult they take on a variety of functions. 



was locomotion ( swimming ) , and all of the 

 primitive Crustacea, such as the fossil trilo- 

 bites, possessed just this type and nothing 

 more. Through the ages it became modified 

 in a most versatile manner in all of the 

 appendages except the swimmerets. For 

 example, the antennules and antennae are 

 receptors for tactile and chemical stimuli 

 and resemble the swimmerets very little. 

 One of the most radical departures is the 

 mandible, or jaw, where there is almost no 

 hint of its progenitor. Among the walking 

 legs, as well as other appendages, the 



mon embryological origin, therefore when 

 two animals show such structures, even 

 though they may not have the same func- 

 tion in the adult form, the animals are 

 known to be closely related. The more 

 nearly the structures are alike the closer the 

 relationship, which means tliat they came 

 from a common ancestor. We shall discuss 

 this topic again in the last chapter of this 

 book. 



Upon cutting through the body wall, 

 among the first and most obvious structures 

 noted are the muscles, particularly in the 



