208 



CHAPTERS ON EVOLUTION. 



of the latter. The anatomical investigation of a Cyclops presents 

 us with an oval body or carapace (Fig. 116, A), bearing a single eye ;. 

 with two pairs of feelers, big and little ; with a jointed tail, forked at 

 its tip ; and with five pairs of swimming feet. In Cyclops-develop- 

 ment a singular resemblance is presented to that of certain low 

 crustaceans parasitic on fishes : and it will be instructive therefore 



to compare these early stages 

 in both groups. The first 

 stage in Cyclops-history 

 (Fig. 122) repeats the now 

 familiar "Nauplius," with 

 its oval body, its central 

 eye, and its three pairs of 

 legs. Next are developed 

 the chest and rail regions ; 

 and six feet appear as the 

 belongings of the latter. 

 Then appears another pair 

 of limbs; and the three 

 limbs of the Nauplius be- 

 come the greater and lesser 

 pairs of feelers, and the 

 great jaws, as in Cypris. 

 After a series of moults, the 

 outlines of the Cyclops- 

 body begin to be apparent ; 

 but it is worthy of remark, 

 that beyond the stage in 

 which the tail-region with 

 its six feet is developed, 

 those lower and parasitic 

 crustaceans the fish-lice 

 just referred to do not 



pass. The further history of Cyclops is simply a record of moults 

 and the growth of new joints and appendages ; that of the fish-lice 

 is a history of retrogression. The fish-lice are represented by such 

 forms as Lernceocera (Fig. 123), or Chondrocanthus, which latter in 

 its maturity maybe found sometimes by the dozen in the gill-chamber 

 of that ungainly fish the Angler or Fishing Frog (Lophiits piscatorius). 

 Lernixocera presents us, as an adult, with a shapeless flattened body, 

 about half an inch long, possessing the merest rudiments of limbs. 

 Each fish-louse begins life as a Nauplius (Fig. 123, B), essentially 

 resembling that of the Cyclops water-flea (Fig. 122). It develops 

 to resemble still more thoroughly the after-stages of Cyclops, but 

 retrogresses therefrom and becomes modified for a parasitic life. 



FIG. 123. FISH-LOUSE AND ITS NAUPLIUS. 



