212 



INVEETEBEATA 



CHAP. 



Nauplius into a primitive Crustacean. The second antenna became 

 gradually shifted forward and lost its masticatory function, while the 

 third appendage became exclusively masticatory and its distal joints 

 shrank to an insignificant palp. At this stage of evolution the 

 animal was assisted in mastication by the modification of the append- 

 ages of the next two or three segments, which formed maxillae, but 

 which were never greatly changed from their original parapodia-like 

 condition. 



When these changes had been effected the ancestor was definitely 

 a Crustacean, and from this level the Ostracoda may well have 

 branched off. In the Ostracod the number of pairs of maxillae varies 

 from one to three in different genera, and what in one genus is a 

 maxilla in a neighbouring genus may be a small thoracic limb. 

 Swimming is mainly performed by the antennae. Finally, in this 

 group alone among Crustacea, there are retained throughout life two 

 pairs of excretory organs, viz., a pair of antennary glands as in the 



higher Crustacea, and a pair of maxillary 

 " shell "-glands as in the lower Crustacea. 



The Cladocera also must have branched 

 off about the same period of evolution from 

 the common stock, and this is true also of 

 some of the Phyllopoda. Those genera, 

 however, like Apus and Branchipus, in 

 which the antennae have lost their swim- 

 ming function, represent the higher stage 

 of development. 



Following the stage of evolution which 

 we have been discussing, a new stage 

 supervenes in which the swimming function 

 began to be handed on to the first thoracic 

 appendages, while the hinder part of the 

 body became specialized to form a rudder 

 by the diminution in size of its appendages. 

 The Copepoda and Cirripedia seemed 

 to have diverged at this point. In them 

 as in the Zoaea larva the appendages 

 of the hinder segments are suppressed 

 altogether a phenomenon doubtless due to 

 the diminution in size, which affects these Crustacea in the same way 

 as it affects the larvae. A condition just previous to this stage is also 

 represented by the Zoaea of Penaeidea and Euphausiadacea, in which 

 a large portion of the swimming function is still carried out by the 

 second antenna. But the process of " handing on " the swimming 

 function to the thoracic appendages, once initiated became progressive, 

 and soon the second antenna became relieved entirely of its swimming 

 functions, which were then exclusively performed by the thoracic 

 appendages, whilst the second antenna was set free for sensory 

 functions. 



FIG. 159. Zoaea larva of the 

 Crab Xantho. (After Cano.) 



d.sp, median dorsal spine of the 

 carapace ; r, rostrum of carapace 

 deflexed and acting as a frontal 

 spine. 



