ARTHROPODS 191 



Abdominal appendages: 



Swimmerets, five pair for swimming and egg attachment. 

 Tail fin, sixth and seventh pairs, backward motion. 

 Study of appendages shows 



1. Modification of similar part, swimmeret, for different uses 



(Homology). 



2. Adaptation for different functions (specialization). 



3. Division of labor among homologous parts. 

 Homology, likeness in structure and origin, shows relationship. 

 Analogy, likeness in function, not necessarily in structure. 



Adaptations of Crayfish for 



1. Locomotion. 



Swimming forward by means of swimmerets. 

 Swimming backward by means of tail fin. 

 Walking either forward, backward, or sidewise. 



2. Protection. 



External skeleton, color, spines, carapace, projecting sides, 

 speed, claws. 



3. Food-getting (what food?) 



Claws, speed, mouth-parts, senses, color. 



4. Respiration (cf. protozoa and worms). 



Gills, adapted by being thin; for osmosis 

 Being well supplied with blood. 

 Being protected; large surface. 

 Water current provided by 

 Gill bailer. 



Locomotion backward. 

 Leg motion in all directions. 



5. Sensation. 



Eyes, ears, feelers, hairs. 

 Life History: 



1. Egg fertilized, attached to swimmeret (protection, aeration). 



2. Hatch in summer, remain attached to mother. 

 3 Grow by moulting (reason). 



4. Top swimmers when young, then on bottom. 



5. Why so many eggs needed. 

 Moulting: 



Reason (cf. internal skeleton). 

 Process: Absorption of lime from shell. 



Carapace splits. 



Water withdrawn from tissues, causing shrinkage. 



Humps out of shell. 



Re-absorption of water and rapid growth. 



Hasty formation of new skeleton. 

 Lost Parts Reproduced: 



What animals can reproduce lost parts? 



Why not so much in higher forms? (Greater specialization.) 



What tissues can man reproduce? 



