THE DECAPOD CRUSTACEA. 



189 



symmetrical, the left side being better developed than the 

 right, and bearing four unpaired appendages used for 

 carrying the eggs (cf. Pagurus). In the male the abdomen 

 is symmetrical, but uncalcified, except for small lateral plates. 



The student should not fail to notice that the four genera 

 just discussed Galathea, Porcellana, Pagurus, Lithodes 

 show in several respects close interrelationship. The point 

 of special interest, however, is that they fall into two sets 

 Galathea and Porcellana on the one side, and Pagurus 

 and Lithodes on the other and that in each set we have 

 a long-tailed (macrurous) form (Galathea in the one and 

 Pagurus in the other) and a short-tailed form (Porcellana 

 arid Lithodes), the brachyurous characters having been 

 acquired independently 

 in the two cases. The 

 fact will serve to illus- 

 trate what is meant by 

 saying that the Deca- 

 pods cannot be logically 

 classified into Brachyura 

 and Macrura, for not 

 only are forms like 

 Galathea and Porcellana 

 transitional between the 

 two, but the brachyurous 

 habit seems to have been 

 acquired independently 

 in several groups, and 

 the rigorous application 

 of the classification must 

 result in the separation 

 of closely allied forms. 

 Thus if we put porce- 

 lain-crabs and the stone- 

 crab together among 

 other crabs, as is often done, we necessarily ignore the fact 

 that they are more closely allied to Galathea and the 

 hermit-crab respectively than to one another. 



The next form to be considered is the pretty little 

 masked crab (see Fig. 54), Corystes cassivelaunus, be- 

 longing to the family Corystidse, which includes only one 



FIG. 54. Masked crab (Corystes cassivelaunus). 

 In part after Herbst. 



