580 



EVOLUTION OF ANIMALS 

 Table 29.1 



Part V 



Paired Appendages of the Crayfish (or Lobster) — Variations of Function on 



THE Theme of a 3-Piece Appendage* 



* The fundamentals of the 3-piece appendage are a basal piece, protopodite, and two 

 branches, an outer one or exopodite, and an inner one or endopodite. Some authorities list 

 20 and others 18 pairs of these appendages, depending on interpretations. The argument 

 for 18 pairs is: that the antennules develop from a structure that is homologous with the 

 prostomium ("upper lip") of annelids not considered a segment; and that the antennules 

 and eyes are basically sense organs, not appendages. A gill is attached to certain of the 

 appendages, is moved as they move and thus washed by more water. 



t Segment indicates the segment of the body represented. 



ij: (Gill) means that a gill is attached to the basal piece. 



ground, the maxillae and maxillipeds holding it while it is crushed by the 

 mandibles. It then passes through the short esophagus to the stomach. Cray- 

 fishes can live in aquaria very well because being scavengers they do their own 

 housekeeping. They seize earthworms and pieces of meat and their chewing 

 competes with modern meat grinders; three pairs of tools hold, cut, shred and 

 grind; all the motions are rapid, including the frequent spitting out of the re- 

 jects. 



The stomach is partially divided into two chambers (Fig. 29.11). In the 

 larger front, or cardiac, chamber there are three hard teeth that form a grind- 

 ing mill moved by muscles attached to the carapace at one end and to the teeth 

 at the other. When the food is crushed fine it enters the pyloric chamber 

 through a strainer of hairlike setae which allow only liquids and fine particles 

 to pass. There it is digested by the pancreaticlike secretion of the large diges- 

 tive glands. In cooked lobsters these are always "liver," green quilted rolls 

 that start arguments, to eat or not to eat. The digested food is absorbed 



