EMBRYOLOGY 



99 



another. In six of these segments are ganglia of the ner- 

 vous system, controlling the action of the muscles of the 

 several segments (Plate 40, A). The crab appears to be 

 very different (Plate 40, }. There does not at first sight 

 seem to be any abdomen at all, but turn the crab on its 

 back, and we see on the under side a small structure cling- 

 ing close to the under side of the body, which when care- 

 fully examined shows the same divisions into segments that 

 we observed in the abdomen of the lobster (Plate 40, B, c). 

 It is the abdomen of the crab, but much reduced in size, and 

 almost functionless. It contains no nervous ganglia and is 

 very different apparently from the abdomen of the lobster. 

 But when we come to study the embryology of the crab we 

 see that it passes through a stage when it has an elongated 

 abdomen with ganglia in six of its seven somites (Fig. 21). 

 This lobster-like stage in the development of the crab is a 

 reminder of the fact that the crab is descended from ancestors 

 resembling the lobster. Let us go a little farther. The 

 lobster has legs like those of a crab, consisting of a linear 

 series of joints. In the embryology of the lobster, however, 

 we find a stage when the legs are double, not single, each 

 leg having two branches (Plate 41, A}. In this regard the 

 lobster larva resembles another member of the group Deca- 

 poda, namely Mysis, a small animal with which many may 

 not be familiar (Plate 41, B and C). We call the stage in 

 the development of the lobster when its legs are biramous the 

 Mysis stage, and conclude that it is an indication that the 

 lobster is descended from Mysis-like ancestors. Some crabs 

 have larvae with biramous legs. Of course conclusions are 

 not drawn from a single indication like the above, but the 

 whole condition of the organism is studied. For the sake of 



