DECAPODS. 461 



of antennae, eyes, jaws, claws, feet, paddles, and tail ; that 

 is, these extremely different organs are only modified 

 locomotive appendages. The voluntary muscles of these 

 animals are composed of transversely striated and per- 

 fectly colorless fibres, and are always inserted on the in- 

 terior of the skeleton, either directly or by means of its 

 processes. Isolated muscles have a ribbon-like form. The 

 nervous system, in its central mass, consists of an abdom- 

 inal cord connecting with the cerebral ganglia by a ring 

 enclosing the oesophagus. In the long-bodied crustaceans, 

 the abdominal cord is composed of numerous ganglia, 

 arranged in successive pairs, from before backwards, and 

 connected by longitudinal commissures. When the body 

 is shortened by the fusion of segments, the number of 

 ganglia diminishes by the coalescence or disappearance 

 of several. The sense of touch is highly developed ; the 

 sense of sight ,is present in nearly or quite all ; but the 

 organs of hearing have been detected only in the highest. 

 The mouth is generally situated underneath and some- 

 what back from the anterior border of the head. The 

 heart is situated in the axis of the body, directly under 

 the shell, at the fore part of the back, and is often at- 

 tached to the internal surface of the skeleton by muscular 

 fibres. The blood is generally colorless. Crustaceans 

 have a wonderful power of repairing injuries to them- 

 selves ; if a leg or other appendage be broken off, an- 

 other like it soon grows in its place. 



Crustaceans may be divided, according to Dana, into 

 three orders, Decapods, Tetradecapods, and Entomos- 

 tracans. 



SUB-SECTION I. 



THE ORDER OF DECAPODS, OR TEN-FOOTED CRUSTACEANS. 



THE Order of Decapods comprises crustaceans which 

 normally have nine cephalic segments, and but five foot 



