1O4 Invertebrata. 



forms existed abundantly ; these fossil forms had three- 

 lobed bodies, and hence are known as Trilobites, and 



they are only found in 



FlG. 5 8. . i rrii 



palaeozoic rocks. Those 

 now living bear gill 

 processes appended to 

 their feet, and hence are 

 known as ' gill-footed ' 

 or Branchiopods. Many 

 closely allied species 

 have the dorsal wall 

 extended in the form 



yclops quadricornis carrying its egg sacs. o f nn ^irp>lr'nrr Ki' 

 f he small figure is the Nauplius or larva. OI E " ( 



valve shell, just like 

 the gill covering laminae in the lobster. 



Those crustaceans which are parasitic are closely 

 related to the water fleas, and undergo retrogression 

 until they become reduced to little sacs with bristles 

 for jaws, with sucker-like fore feet, and often with no 

 trace of segmentation (fig. 59. A), Some live on the 

 bodies of larger crustaceans such as lobsters, others 

 on tunicates, but they are mostly found attached about 

 the gills of fishes. The early stages of these are little, 

 free, marine Nauplii or larvae, with developed jaws 

 and a moderate post-abdomen (fig. 58). Many non- 

 parasitic species remain for their whole life in a state 

 like that of the larvae of these parasites. 



In all these lower crustaceans the earliest stage 

 of existence after emission from the egg is in the 

 form of a minute oval body with three pairs of limbs 

 and one central eye. This is known as a Nauplius, 

 and it assumes its adult form by the growth of new 

 segments and new limbs. The nauplius stage of 



