INVERTEBRATE ZOOLOGY. 115 



fonu, and must be compared with it, part by part. The most common modi- 

 fications of the tj'pe are the following : — 



(a.) Excessive development of a part. Ex. The tergite of the first seg- 

 ment of the body. In some species this extends beyond the end of 

 the abdomen. 



(b.) Reduction of g. part. Ex. The sternite of the same segment. The 

 reduction may lead to complete loss. 



(c.) Change of form to subserve a certnin function. Ex. The antennae, 

 jaws and legs are all appendages. 



(d.) Fusion of several parts. This is done wherever support is needed, 

 either for some powerful muscle, or against extensive injur}-. Ex. 

 The partial fusion of the sternites in the thorax, or the complete fu- 

 sion of the head to protect the brain. This change often affects the 

 somites themselves so that their identification is often difficult. 



Q I , Rides for resolution of fused somites. 



1. As each pair of appendages represents a somite, tlieir niimber will 



correspond to the minimum number of somites. Why the mini- 

 mum ? This is called the " Rule of Savigny." 



2. Often internal parts retain metameric structure longer than external. 



Why V Which parts are moi-e directly under the influence of exter- 

 nal condititms ? The nervous system is especially piimitive in this 

 regard. 



3. Individual develojiment records past conditions more or less perfectly, 



and thus parts fused in the adult may be sejjarate in the embryo or 

 the larval form. 



92. Special .^tndy of e.vternal details. Place a specimen upon the stage 

 of a dissecting microscope and examine with the lowest lens, controlling the 

 work with a pair of forceps in the left hand and a stout needle in the right. 

 For minute details use the higher lens. For some pui-poses the lens is best 

 removed from the microscope and used like a hand lens. If a specimen has 

 just been removed from a bottle of alcohol it should l)ecome dry before the 

 investigation to avoid the confusing refraction from the liquid. 



1. Head. Remove this from the body, place it face towards you upon 

 the stage and notice : — The antennae, delicate organs of toiich. How 

 many joints? The two large eyes at the sides. These are ■•com- 

 pound," composed of numerous facettes. Look at these with the 



