CRUSTACEA 57 



dence of segmentation only on the ventral side. The 

 thoracic appendages are specialized; most of them have lost 

 the outer branch and become uniramous. Yet with careful 

 examination in the laboratory we can demonstrate that a 

 fundamentally similar plan of structure is shown by all 

 of the appendages on the body of the crayfish, except the 

 eye. If time permitted it would be possible to show many 

 other ways in which the crayfish is specialized, but we must 

 pass on to consider some of its relatives. 



CLASSIFICATION OF CRUSTACEA 



The Crustacea have in their specialization followed various 

 lines and we can divide the class into two great groups, or 

 subclasses, and a large number of orders. It will probably 

 not be profitable for the student to try to learn the follow- 

 ing list, but it is given in full in order that one may be able 

 to see for himself what a degree of diversity has been at- 

 tained in ihigL most primitive class of arthropods, contain- 

 ing only about 16,000 living species. Before we pass to 

 the table of classification it will be well to point out the 

 chief lines which specialization has followed : 



1. The heart has been greatly shortened so that in some crustaceans 

 it occupies only one or two metameres. 



2. The kidneys are always restricted to one pair. 



3. Gills may be developed along the sides of the thorax, or on the 

 thoracic or abdominal appendages. 



4. The primitive biramous appendage is frequently modified by the 

 dropping out of the external ramus or by the loss of segments. 



5. A carapace may be developed. This may cover only a part or 

 the whole of the thorax, or be large enough to enclose the entire body. 



6. There is a tendency to acquire mouth parts at the expense of the 

 walking legs. 



7. The appendages are often lost on the abdomen. 



8. The eyes sometimes become stalked. 



With these points in mind let us pass on to the classifica- 

 tion of the Crustacea. 



Trilobita. This group of extinct arthropods was ex- 



