364 



ANIMAL LIFE-HISTORIES 



there is a protective armour of shelly plates, between which and 

 the body is a considerable space that is made use of as a brood- 

 pouch. The numerous eggs are glued, as it were, into relatively 

 large masses, which are attached to a pair of triangular flaps. 



Among the little Mussel - Sh'imps 

 iyOstracoda) it appears that the eggs may 

 either be attached singly to water-plants, 

 as in the commonest freshwater member 

 (Cypris) of the group, or else, as in many 

 marine forms {Cypridina, &c.), sheltered 

 within the bivalve shield of the mother 

 until such time as they hatch out. 



In all the above groups of Lower 

 Crustaceans, Mussel - Shrimps and most 

 Water - Fleas excepted, development is 

 indirect, the young animals hatching out 

 in the form of a Nauplius larva (fig. 887) 

 with oval unsegmented body and large 

 unpaired eye in the middle of the fore- 

 head. The swimming organs are three 

 pairs of appendages, which afterwards be- 

 come the small feelers (aiitenmtles), large feelers [antemiix), and 

 first jaws (^mandibles) of the adult. Many authorities at one time 

 thought that the Nauplius repeats in its main features the 

 characters of the simple ancestral forms from which all Crus- 

 taceans may be imagined to have taken origin, but this view is 

 now practically abandoned. There can be little 

 doubt that Crustaceans are the specialized descen- 

 dants of animals resembling the recent Bristle- 

 Worms, and an instructive comparison may be 

 made between the Nauplius and the typical 

 Trochosphere larva (p. 359) which is commonly 

 an early stage in the life-history of such worms. 

 If a Trochosphere were to lose its cilia and 

 grow three pairs of appendages it would closely 

 resemble a Nauplius, and evolutionary changes of the kind are 

 quite conceivable, for such appendages would prove more efficient 

 locomotor organs than the cilia which they superseded, and we 

 are justified in making the general assertion that evolution always 

 tends towards greater efficiency, i.e. to an increasingly perfect 



Fig. 886. — Female Cyclops with egg- 

 bags, enlarged, ant. i and ant. 2, 

 Antennules and antenna;. 



Fig. 887.— Nauplius 

 Larva, enlarged 



