296 



PHYLUM ARTHROPODA. 



a 



resemble other Arthropods; as special characteristics we 

 notice the two pairs of antennas, the presence of carbonate 

 of lime in the cuticle, and the nature of the respiratory 

 organs these, with few exceptions, being adapted for breath- 

 ing in water. While these characters remain constant 

 throughout the group, there is an almost infinite variation 

 in detail. In regard to the segmentation of the body, we 



notice that, apart from 

 the general tendency 

 to reduction which is 

 so marked in many 

 parasitic forms, the 

 higher forms as com- 

 pared with the lower 

 show marked special- 

 isation. In the primi- 

 tive Phyllopods the 

 body consists of a 

 large but varying num- 

 ber of segments, re- 

 markably uniform in 

 structure. The higher 

 Crustacea, on the other 

 hand, are characterised 

 by their relatively few 

 but constant segments, 

 which exhibit marked 

 division of labour ; a 

 comparison of Nebalia^ 



m 



FlG. 143. Nervous system of shore crab 

 (Carcinus mcenas}. After Bethe. 



/>;-., The supra-cesophageal mass; ^., gullet 

 surrounded by gr. , the gullet ring; ;., the 



sub-cesophageal mass representing a fusion SchlZOpods and 

 of the thoracic ganglia of the crayfish, and -, 



giving off nerves to the limbs ; behind it is pOClS, a SCriCS WHICH 

 a short strand representing the abdominal illustrates the develop- 

 ganglia of the crayfish. a 1 ., antennules ; f, 



a2., antenna; ; e., eye. ment Of the thorax, 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 appendages, like the parapodia of 

 Annelids, are primitively organs of locomotion, usually 

 adapted as swimming organs. In Phyllopods the great 

 majority of the appendages remain permanently at this 

 level. It is worth notice that in the Nauplius and in 



