CRUSTACEAN CHARACTERS 



355 



comparison of Nebalia, Mysis, Euphausia, Penaus, 

 Nephrops, will make this plain. The same gradual process 

 of specialisation is observable in the appendages. Typically 

 consisting of a basal piece and two branches, the append- 

 ages, like the parapodia of Annelids, are primitively organs 

 of locomotion, usually adapted as swimming organs. In 

 Phyllopods the great 

 majority of the append- 

 ages remain perman- 

 ently at this level. It 

 is worth notice that in 

 the Nauplius and in 

 Ostracods and in free- 

 swimming Copepods, 

 the antennae themselves 

 are swimming organs. 

 Just as, however, in the 

 Annelid head the loco- 

 motor function of the 

 parapodia becomes 

 subordinated to the 

 sensory one, so also in 

 Crustacea the anterior 

 appendages of the head 

 become specialised as Fig. 199. — Nervous system of shore-crab 

 sense organs. Again, {Cardnus m^na5).— After Bethe. 



the appendages in con- *''•. The supra-cesophageal mass; g., gullet 



nection with the mouth 

 become modified in 

 connection with ali- 

 mentation, and the 

 further processes of 

 specialisation which differentiate the regions of the body 

 are reflected in the appendages of these regions. It is this 

 specialisation of certain appendages to function as mastica- 

 tory organs which especially characterises Arthropods as 

 compared with Annelids. 



In the nervous system there is always a certain amount 

 of fusion of ganglia — these never being so nunierous as the 

 segments — but the fusion is more marked in the more 

 specialised forms. In the Crabs the ventral chain is repre- 



surrounded by g.r., the gullet ring ; m., the 

 sub-cesophageal mass representing a fusion of 

 the thoracic ganglia of the crayfish, and 

 giving off nerves to the limbs ; behind it is 

 a short strand representing the abdominal 

 ganglia of the crayfish. a^., Antennules ; 

 a'"., antennae ; e., eye. 



