THE ROCK-BARNACLE 77 



attached base formed by the head and antennae, a stalk, 

 which for a short time is flexible, and an enlarged free end, 

 which bears the jointed legs, and is defended by valves 

 and plates. Provision is now made for the protection of 

 the body by an outer case. A fold of skin is pushed out, 

 which grows rapidly until it envelops the body on all sides. 

 Hard plates, answering in a general way to the dorsal and 

 ventral plates of a free-swimming crustacean, form upon 

 this fold. They soon lose all traces of a serial arrangement, 

 and become disposed in an outer and an inner circle. The 

 inner circle sinks within the outer one, and constitutes the 

 valves described above, whose free edges gape to allow 

 the protrusion of the limbs. 



It is easy to see that the barnacles, whether stalked or 

 sessile, derive important advantages from the possession 

 of early free-swimming stages. We might go further, 

 and say that they could not continue to exist without 

 motile stages of some kind, for a ready mode of dispersal 

 is indispensable to the due spacing of all animals which as 

 adults are stationary. We learn from observation of the 

 course of development of various animals that three con- 

 ditions favour the production of special motile stages. 

 These are (i) a sedentary life in the adult stage ; 

 (2) heavy armour ; (3) a marine, and especially a littoral 

 habitat. All three conditions are associated more or less 

 intimately, and often co-operate to produce the same result. 

 Sedentary life in the adult favours mobility in the larva, 

 for if the adult cannot migrate, the young must do so. 

 Heavy armour makes the adult more sedentary. Habitat, 

 little as it seems to be related to modes of dispersal, is 

 really closely connected therewith, for the waters of the 

 sea, which are continuous over vast areas, and kept in 

 circulation by wind- currents, offer great facilities for early 

 migration. Rivers are much less favourable. They are 

 relatively small and isolated, and they end in the sea, 

 which destroys at once nearly all kinds of freshwater 

 animals which enter it. The crowded state of the shallow 



