162 LIFE BY THE SEASHORE. 



legs ; but in a crab where all the legs display this peculiar 

 modification, any one of them may be thrown off. Separa- 

 tion always takes place at the one point, and the fusion 

 of segments is to be regarded as a special adaptation to^ 

 facilitate this autotomy or self-mutilation. In this respect 

 also, therefore, the crab shows an increase of specialisation 

 as compared with the lobster. 



Turning now to the ventral surface of our crab we find 

 that, as already noticed, the rudimentary abdomen is flexed, 

 and lies along the ventral surface of the thorax. But it is 

 much narrower than the thorax, and the lateral insertion of 

 the legs exposes the ventral surface of the latter much more 

 fully than in prawn or lobster ;. so we find in the first place 

 that this ventral surface is in the crab very firm and hard 

 completely calcified. Bend the abdomen gently backwards, 

 and you will see that the thorax has a deep ventral groove 

 in which the abdomen habitually lies. The abdomen itself 

 bears rudiments of appendages, but these are much reduced. 

 Let us recall for a moment the abdominal appendages of 

 prawn and lobster or crayfish. In the prawn there are six 

 pairs of functional swimmerets, the last pair being much 

 the largest. In the lobster the first pair is rudimentary in 

 the female, and curiously modified in the male ; the next 

 four pairs are small and of little use in swimming, though 

 in the'female they carry the eggs ; the last pair is large, and 

 forms with the telson the powerful tail fan. In the crab, 

 with the reduction of the abdomen, we have the total 

 suppression of this tail fan, and the development of the 

 others varies in the two sexes. In the male the two 

 anterior pairs only are present, and are much modified ; in 

 the female four pairs are present; they are long and 

 delicate, and furnished with numerous hairs. As in the 

 lobster they are used for carrying the eggs. The number 

 of segments in the abdomen of crabs tends to be reduced, 

 more especially in the males. 



We have thus briefly revised the main points of external 

 structure in three types of Decapods, and may look for a 

 little at the order in general. We have already noticed the 

 striking resemblances between prawn and lobster which 

 have led naturalists to classify them together as Macrura, 

 or long-tails, in contradistinction to the short-tailed crabs, 



