METAZOA CRUSTACEA. 355 



pairs of walking-legs, one pair of arms, six pairs of 

 mouth parts, two pairs of antennae, and one pair of eye- 

 stalks although the same in number as in the lobster, 

 are crowded together. The maxiliipeds have lost their 

 foot-like appearance, but are distinctly mouth organs, 

 the third pair being folded like a lid over the other 

 pairs. 



The internal structure as well as the external parts 

 reveal remarkable concentration. The internal skeletal 

 portions of the cephalothoracic segments are consolidated 

 (No. 866, vertical section), while the carapace, extending 

 backward on the dorsal side, covers them completely 

 from view. The nervous system, especially, is consolidated 

 in the cephalothorax in a most surprising manner. No. 

 867 exhibits the great nerve mass in the center of this 

 region with the nerves radiating from it to the different 

 parts of the body. The stomach and intestine belonging 

 to the digestive system are well seen in No. 868. 



We have already seen that the Crustacea offer ex- 

 tremely interesting modifications of structure brought 

 about by the varied habits of the animals. There still 

 remain in the Synoptic Collection numerous examples of 

 Brachyura which have been chosen to illustrate more 

 fully the instructive adaptations of this group. 



Callinectes hastatus Ord,the "edible," "blue," or "soft- 

 shelled" crab (No. 869 ; a, male ; b, female) can swim 

 rapidly, and for this purpose the last pair of legs is con- 

 verted into a pair of flattened paddles. These are such 

 strong organs that they are able to propel the body for- 

 ward in spite of its unwieldy proportions. 



The abdomen of the male tapers abruptly to a mere 

 vestige of its former condition and lies locked by two 

 teeth into a deep groove in the ventral side of the cepha- 

 lothorax. The abdomen of the female, on the other 

 hand, is broad and adapted for carrying the eggs. The 

 sharply notched carapace and the spiked arms of this 

 genus offer a strong defence against its enemies. 



