18 



In all Bracliym-a, liowever, the basal joints of the antennulai* peduncles are 

 separated from one another by an inter-antennular .se])twm formed by the union of 

 the front with the epistome. 



The 3/T? pair of appendages of the head are the antcnnse, which differ from 

 those of Neplirops in never having a movable antennal scale, and in the reduced 

 leno-th of the flagellum : moreover owing to complete fusion between the 2nd and 

 3rd joints (which form the so-called " basal joint ") the antennal peduncle consists 

 of only four joints, of which the 1st, or true basal joint, is, in crabs above the 

 level of the Bromides and Raninidse, reduced in size, fixed, and more or less con- 

 cealed. There are a good many crabs, especially among the Oxystomes, in which 

 the antennae are quite rudimentary. 



There are certain Maciuia {e.g., t.he Loiicates) and plenty of Anomura, in whicli the 2ncl and 

 3rd joints of the antennal peduncle are fused to form a single piece and the size and mobility of 

 the 1st joint are considerably decreased. 



The 4>th, Uh and Qtli pairs of appendages of the head are, respectively, 

 mandibles and two pairs of maxillge not essentially different from those of 

 Nephrops. The fold of the carapace in Avhich these appendages, along with the 

 three pairs of maxillipeds, lie is deep and is, usually, very distinctly dehmited both 

 at the sides an'd in front : it is known as the buccal cavern, and its roof — i.e., the 

 part that Hes between the epistome and the labrum — is known as the cndostonte 

 or " palate." 



The appendages of the tliora.v do not essentially differ from those oi Nephrops : 

 the first three pairs are foot-jaws or maxillipeds, the fourth pair are great 

 chelipeds, and the last four pairs are , ambulatory legs which are never truly 

 chelate though sometimes the last pair or last two pairs may be subchelate. 



The !.•</ and 2nd maxillipeds resemble those of Nephrops, but in the 1st 

 maxilhpeds the endopodite and epipodite are usually much larger, and in the 

 2nd maxilhpeds the exopodite is usually longer and broader and the podobranch 

 is usually well developed. 



The 3rf?- or external maxillipeds have, in the great majority of crabs, quite 

 lost their pediform character and become broad valves or folding-doors that 

 completely close the broad buccal cavern in which all the other mouth-parts lie, 

 so that these are all concealed from view. This change in the character of the 

 external maxillipeds is mainly brought about by a broadening and flattening of 

 the ischiopodite and meropodite : the three terminal joints ttsually, but by no 

 means always, remain small and form a sort of "flagellum" or "palp" to the 

 enlarged meropodite. The exopodite, which is sometimes six>ken of as the 

 exognath, may either remain comparatively slender or may become as much 

 broadened as the ischiopodite and meropodite. 



There are, however, many crabs in which the external maxillipeds are almost as distinctly 

 pediform as they are in Nephrops, and do not conceal the other appendages of the mouth : in 



