THE DECAPOD CRUSTACEA. 161 



but its inner branch, instead of being leg-like, is converted 

 into a flattened plate, covering over all the anterior ap- 

 pendages, and closing the anterior opening of the gill- 

 chamber. In Mysis, a simpler Crustacean than any of those 

 yet considered, all the eight thoracic appendages are similar, 

 all consist of a basal piece with a leg-like inner branch and 

 a slender outer branch. In prawn and lobster the anterior 

 three only retain this " biramose," or two-branched structure, 

 but they otherwise generally resemble the walking legs 

 proper, this being especially true of the third. In the crab 

 these three maxillipedes are fundamentally modified to sub- 

 serve functions connected with respiration and mastication, 

 and the structural gap between them and the true legs 

 attains its maximum. It is facts of this kind which induce 

 morphologists to regard the crab as more specialised than 

 the lobster, though it has lost some of the powers which 

 the latter possesses. 



The legs of the crab will be found to display many in- 

 teresting peculiarities. The first pair are always the largest, 

 and constitute the main weapons of offence and defence. 

 Their shape and markings are often characteristic of the 

 species, and in many cases they fit in repose very closely to 

 the margin of the carapace, a point we shall consider later. 

 Near their base is the slit through which water enters the 

 gill-chamber ; a carapace which has been removed with 

 sufficient care not to damage the movable gill-cover will 

 show a notch at this point. The remaining four pairs of 

 legs never bear forceps, and differ markedly in the shore 

 crab and the edible crab. They always form the organs of 

 locomotion, and are inserted laterally so as to form an 

 efficient support for the body. The last pair arises some- 

 what dorsally. An interesting point about them is that all 

 are made of six pieces only. In the lobster the chelipeds, 

 or great claws, have six joints, the other. legs seven. A 

 careful comparison will show that this is due to the fact 

 that in the great claws segments two and three, counting 

 from the base, are fused together, the line of junction being 

 clearly marked. When a lobster throws off its great claws, 

 as it often does when frightened or molested, separation 

 takes place at this junction line. A lobster only possesses 

 the power of throwing off its great claws, and not the other 

 M 



