Io6 Invertebrata. 



Some of the metamorphoses of crustaceans are 

 strange ; in none more so than in the barnacles and 

 acorn-shells (figs. 4 and 5, pp. 9 and 10), the lowest 

 subclass of the series. Acorn-shells or Balani, are the 

 little limpet-like shells which encrust the rocks along 

 all our coasts and which can be at once recognised by 

 the opening at the top of the conical shell, which is 

 closed by the lateral valve-like or beak-like plates. 

 Barnacles are commonly found adhering to logs of 

 wood or to ships' bottoms. These begin life as active 

 nauplius-like larvae, which every autumn are to be 

 found swimming along our coasts. This larva at its 

 early moults develops a lateral mantle-fold. At its 

 fourth change in shell, the front of its head becomes 

 fixed by the flattening of one of the joints of the 

 antennae and by the secretion poured out by a gland 

 which, though placed in the body, has its duct open- 

 ing in the altered joint of the antennae. At the fifth 

 moult the eyes and antennae vanish, the head becomes 

 fixed by a broad base of attachment, the mantle-like 

 fold of integument surrounds the body and becomes 

 calcified into a shell of many valves, within which the 

 hinder parts of the body are enclosed together with 

 their six pairs of limbs. These limbs remain free 

 and capable of slight protusion, while the mouth 

 with its mandibles lies at the bottom of the mantle 

 cavity. 



Some Balani select curious places of residence. 

 Coronula lives on the skin of the whale ; Anelasma 

 often is adherent to fishes, and many others to corals. 

 One closely allied group of degraded forms are para- 

 sites on the abdomen of crabs. To the sub-class 



