VIII 



AKTHEOPODA 



205 



that the Crustacean ancestor was like a Polychaete Annelid, with 

 fairly numerous segments bearing somewhat feeble and membranous 

 appendages like parapodia, but possessing greatly enlarged append- 

 ages attached to the first three segments, which fulfilled the major 

 part of the work of locomotion, the first pair of these appendages 

 alone having passed in front of the mouth. Corresponding to the 

 diminution in size of the whole body in the larva as compared with 

 its original size in the ancestor, the comparatively functionless 

 posterior appendages have been suppressed. 



But from such a form, with only one pair of appendages in front 



caudf 



FIG. 151. Dorsal aud ventral views of the " Copepodid " larva of Adheres ambloplitis. 

 (After Wilson.) 



A, dorsal view. B, ventral view, 0/1, first antenna ; of 2 , second antenna ; cauil.f, caudal fork ; 

 fr.gl, frontal gland ; lab, labruin ; inn, mandible ; mx*, first maxilla ; mx 2 , second maxilla ; mxp, maxilli- 

 pede ; th, thoracic legs ; u.l, under lip. 



of the mouth, with the two next pairs in the form of powerful 

 locomotor organs not specialised for either mastication or sensation, 

 the Arachuida can be derived ; and the fact that Onychophora and 

 Insecta likewise have only one pair of antennae shows that they too 

 could be traced back to such an ancestor. Finally, the extinct 

 Trilobita, whose jaws bear long forked palps, and which possess only 

 one pair of antennae, seem clearly to belong to the same cycle of affinity. 

 That this reasoning is justifiable and not far-fetched we may 

 illustrate by taking a case where we may almost say that the 

 ancestor is known, and where we are therefore in a position to 

 compare the ancestor and its representation in the larva. This is 

 the life-cycle of the parasitic Copepod, Adheres ambloplitis, which 



