176 ANIMAL METAMORPHOSIS. 



Many of the Crustaceans make their appearance from the egg 

 as a minute, rounded, oval, or pear-shaped larva, with three 

 pairs of legs — the anterior simple, the others bifid. It is called 

 a Nauplius (Figs. i8, 22, 24, 25). In front, above the mouth, 

 lies a single, simple eye. It swims freely about, and by various 

 moultings and changings gradually assumes the adult form. 

 The Epizoa, Cirripedia, and Entomostraca, all commence life in 

 the Nauplius form, and the primitive form of one of the shrimps 

 (Peneus) has also been shown, by Fritz Muller, to be a Nauplius 

 (Fig. 18), which at first has only three pairs of limbs, but soon 

 acquires additional pairs and a jointed body (Figs. 19 and 20), 

 and gradually approximates more and more to the adult form from 

 which it originated. 



Here, and with the Entomostraca, a gradual development 

 takes place to a more complex structure, but in other Crustaceans 

 which take on a parasitic condition we find that the Nauplius, 

 instead of proceeding to a higher form, takes on a kind of retro- 

 gressive metamorphosis. Thus, in the case of the Saccidina (Fig. 

 21), a parasite which infests crabs, and is a mere sac filled with 

 eggs and absorbing nourishment from the juices of its host by 

 root-like processes, and in the Ler?i(Eocera (Fig. 23), which, in its 

 adult condition, is an elongated, worm-like creature found attached 

 to the gills of fishes, the early, active Nauplius forms (Figs. 22, 24) 

 lose their locomotive and sense-organs, and degenerate into 

 comparatively simple bodies, leading an almost vegetative life, and 

 becoming mere re-productive machines. 



In the Cirripedia — the Acorn-shells and Barnacles — a similar 

 retrogression takes place. The Acorn-shells (or Balani) are the 

 limpet-like shells which encrust the rocks along our seashore, 

 occurring in myriads upon every solid object between the tide- 

 marks 3 while the Barnacles (Fig. 26) are commonly found 

 attached by an elongated stalk to floating logs of wood or ships' 

 bottoms. They commence life in the active Nauplius form, 

 which every autumn can be found swimming in large numbers 

 along our coasts. It has the usual three pairs of legs, the first of 

 which is uncloven, the two subsequent pairs forked ; the back is 

 covered with an ample shield, often terminating anteriorly in two 

 extended horns, while in the Nauplius of the barnacle (Fig. 25) 



