390 



COMPARATIVE ANATOMY 



CHAP. 



and pleopoda are originally biramose and in this animal retain this character during 

 litL'. Xo great importance should be attached to the names of the developmental 

 stages [CiiJyptopis, Furcilia, Cyrtopia) ; they are referable to a time when the larvfe 

 so "named were thought to be different genera. "We thus again see, from the 

 larval history of the Euphaiisida:, that the body with its limbs is differentiated 

 from before backward. We notice, however, special and important exceptions 

 to this rule. In the first place the rudiment of the last pair of pleopoda 



Fig. 262.— Larvse of Euphausia. A^ Nauplius, last form before moulting (after MetsclinikofF). 

 _B, Protozoaea. C, the same somewhat older (after Glaus), i/t, Thoracic segments ; n?;, abdomen ; 

 (rt^-ag), abdominal segments ; Oii, anus ; /s, frontal sensory organ; 1-5, limbs of the head ; ag, 6th 

 pair of pleopoda. 



appears before those of the other pleopoda, before even those of the thoracic 

 feet. This is noteworthy on account of the special form of and the important 

 part played by these pleopoda as part of the caudal fin in the older stages of develop- 

 ment and in the adult. We further note that although the thorax becomes 

 segmented sooner than the abdomen, and although on the thorax as on the abdomen 

 the extremities become differentiated in succes.sion from before backward, the rudi- 

 ments of the extremities on the thorax and the abdomen are almost simultaneous 

 and sometimes they even occur earlier on the abdomen. 



