12 A BIOLOGY OF CRUSTACEA 



mantle become more regular. Some other stalked barnacles lose 

 the mantle plates completely. A second line of evolution can be 

 traced which involves the loss of the stalk to form acorn 

 barnacles. Catophragmus is a stage which might be reached by 

 reducing the stalk of Pollicipes; eight large plates are surrounded 

 by a series of smaller scales. From a form with eight plates one 

 can derive various others by the loss of some plates and the joining 

 of other plates (fig. 11). Some cirripedes have became parasites; 

 these are dealt with in chapter 8. 



The various groups that have been dealt with so far have varied 

 greatly in the number of segments constituting the body. In the 



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Fig. 12. Palaemon ( = Leander) serratus (Decapoda Natantia). 



Adult female viewed from the right side. Actual length, from eye 



to tip of telson about 6 cm. 



Malacostraca the segmentation has settled down, and the various 

 segments can be aligned with one another throughout the mala- 

 costracan orders. There are eight thoracic and seven abdominal 

 segments, though as a rule only six abdominal segments are in 

 evidence. It is a general rule that the male ducts open on the last 

 thoracic segment and the female ducts open on the sixth thoracic 

 segment. Within the limitations of this basic plan there has been 

 great radiation into different groups, and a general increase in body 

 size when compared with other Crustacea, so that the Malacostraca 

 are now abundant, widespread and quite the most conspicuous of 

 all the Crustacea. 



The primary branchings in the radiation of the Malacostraca 



