47 



each metamere. Renal organs were present in the form of 

 paired nephridia. In the ancestral Crustacean, derived from 

 this primitive segmented worm, the lateral appendages must 

 have become developed into more efficient locomotory organs, 

 by their elongation and the formation of joints upon them. 

 The increased power of movement which this change con- 

 ferred probably resulted in an increase in the muscular 

 system connected with the appendages, and this, in order 

 that the new organs might be fully taken advantage of, would 

 require the formation of a hard skeletal tissue, to which the 

 muscles might be attached. To meet this want, the more or 

 less calcified cuticular *' shell " of the Crustacea was devel- 

 oped, and, in the higher members of the group, has been 

 brought to a high degree of perfection as an exoskeleton. 



The further changes which have taken place during the 

 evolution of the Crustacea, have been mainly in the direction 

 of producing " heteronomy " in the segments of the body. 

 Division of labour amongst the appendages has resulted in 

 great modifications in structure, some being converted into 

 jaws, others into sense organs, walking legs, swimming legs, 

 and other organs ; while the segments themselves have also 

 undergone change, the exoskeleton becoming in many cases 

 fused over large tracts to form a continuous hard covering or 

 carapace. Some of the metameres at the front of the body 

 more or less completely unite with the prse-oral lobe to form 

 a distinct " head." 



The free larval stage known as the Nauplius, is of such 

 constant occurrence throughout the Crustacea, though exhib- 

 iting slight peculiarities in each group, that it is almost 

 certain that it represents more or less closely a far-back 

 common ancestor. The Nauplius of the Phyllopoda (fig. 17), 

 the most nearly primitive group of Crustaceans, is an ovate 

 body, provided with three pairs of appendages, attached to 

 the anterior (cephalic) region. These appendages afterwards 



