THE CLASSIFICATION OF CRUSTACEA 43 



limpet, with a hole at the top which is closed by 

 four movable valves. 



The Stalked Barnacles, like Lepas (suborder 

 Pedunculata), and the Sessile Barnacles, or Acorn- 

 shells, like Balamis (suborder Operculata)^ together 

 form the order Thoracica. Of the other orders 

 which compose the subclass Cirripedia, the only one 

 that need be mentioned here is the Rhizocephala, 

 which comprises strangely degenerate parasites living 

 on other Crustacea. 



The Cirripedia are unlike nearly all other Crus- 

 tacea in the fact that, with few exceptions, they are 

 hermaphrodite, having both sexes united in each 

 individual. In certain species of the Stalked Bar- 

 nacles, however, there are minute male individuals 

 that are attached, like parasites, to the large her- 

 maphrodites. In a few species the large individuals 

 only possess female organs, so that the separation of 

 the sexes is complete. 



The remarkable larval metamorphoses of Cirripedes 

 and the modifications of structure presented by some 

 parasitic forms will be described in later chapters. 



The fifth and last subclass, that of the Malacos- 

 TRACA, is by far the largest and most important, and 

 will require to be considered in more detail than any 

 of the others. The animals composing the various 

 orders into which the subclass is divided differ very 

 greatly in structure, but they all agree in having 

 typically the same number of appendages as the 



