THE EVIDENCE FROM DEVELOPMENT. 207 



are cast aside as useless. The process of extreme modification for 

 a life of parasitism effectually moulds the remaining features of the 

 organism in the characteristic ways of Sacculina life namely, as the 

 sausage-like sac, fixed to its crab-host. But there can be no question, 

 that Barnacle growth and Sacculina development run in strictly 

 parallel grooves. 



Allusion has been made to the likeness exhibited by the " pupae " 

 -of the Barnacle and Sacculina to the perfect and adult form of those 

 water-fleas, which, like Cypris and Daphnia (Fig. 116, B, c), are 

 familiar tenants of our fresh waters. The development of -the " water- 

 fleas " under which general name very diverse beings are included 

 is highly instructive, in that it leads us to note how the community 

 of development existing among Crustacea 

 -extends its roots so as to include every 

 group or order of that class within its limits. 

 The Cypris (Fig. 116, B) and its neighbours 

 .are known by their possession of a distinct 

 bivalve shell that is to say, a shell con- 

 sisting of two pieces, united along the back 

 by a membrane serving as a hinge. Two 

 or three pairs of feet exist, but these 

 creatures appear to swim chiefly by aid 

 of the tail. Now, the young Cypris leaves 

 the egg as a Nauplius " with three pairs FlG ' I22 - NAUPLIUS OF CYCLOPS. 

 of limbs. It possesses, like the Barnacle -nauplius, a single eye, 

 and it appears to develop a shell likewise. The adult condition 

 is attained in due course, with the production of the bivalve 

 shell ; and the three pairs of limbs of the " Nauplius " are con- 

 verted respectively into the greater and lesser pair of antennae and 

 into the mandibles or jaws of the adult. The other feet of the full- 

 grown Cypris are also developed in its later stages of growth, which 

 .are manifested by frequent moultings of the skin. A young Cypris 

 therefore resembles a young barnacle in its Nauplius-form, and in the 

 transformation of its anterior limbs into antennae or feelers, which, in 

 the water-fleas, serve the purpose indicated by the latter name or may 

 even be used for swimming, as in the Daphnia, or " branch-horned 

 water-flea " (Fig. 1 16, c). In the correspondence between the bivalved 

 Cypris and the pupa Barnacle or pupa Sacculina, we may possibly 

 discover, likewise, the ultimate point of divergence between these 

 diverse groups of Crustaceans. 



Other water-fleas, such as Daphnia and Cyclops (Fig. 116, c, A), 

 present variations in their early history from the chronicle of Cypris 

 development. The Cyprides are perhaps the least modified of the 

 water-flea race ; this conclusion being supported by the greater com- 

 plexity of other water-fleas as well as by the course of development 



