THE DECAPODA 257 



degree of mobility. In the higher forms they become firmly united, 

 with the exception of the last thoracic sternum, which may be 

 movable (Astacidae, Parastacidae, and Anomura). 



In front of the mouth, regions representing the sterna of the 

 three preoral " somites " can be distinguished, but on account of 

 the " cephalic flexure " the ophthalmic and antennular sterna are 

 directed forwards, or even upwards. The antennal sternum is 

 mainly represented by the epistome, a plate of varying shape lying 

 between the labrum and the bases of the antennae. In the 

 Natantia the epistome is comparatively narrow, and on each side is 

 separated from the lateral portions of the carapace by the exhalent 

 branchial channels. In most of the Reptantia the epistome (Fig. 

 147, A, ep) is broad and comes in contact with the carapace on each 

 side, and in the Brachyura it becomes firmly united with it. In 

 this way there is defined more or less distinctly a buccal frame 

 within which lie the mouth-parts, and which in most Brachyura is 

 closed by the operculiform third maxillipeds. The sides of this 

 buccal frame are formed by the free antero -lateral margins of 

 the carapace (Fig. 147, B, l.m), while in front it is more or less 

 distinctly delimited by the epistome itself, or by a transverse ridge 

 (Fig. 147, B, a.m) which divides the epistome into two parts, the 

 epistome proper and the endostome or palate (end). In most 

 Brachyura also (except the Dromiacea) the proximal segments of 

 the antennae are fused with the epistome. In the Macrura the 

 anterior margin of the carapace forms on each side of the base of 

 the rostrum a more or less distinct " orbital notch," within which 

 the eye rests when it is turned outwards. In the Brachyura this 

 transverse direction of the eye-stalks is permanent, and the orbit' 

 is usually (except in Dromiacae) completed by the downgrowth 

 of a process (l.p) from the front, external to the antennules, which 

 unites either directly or, more usually, by intervention of the second 

 segment of the antenna, with the sub-orbital lobe (s.o) of the cara- 

 pace. Further, in all .the Brachyura the rostrum or frontal plate 

 sends downwards in the middle line a process (m.p) which unites 

 .in front of the ophthalmic and antennular sterna with the epistome, 

 and separates from one another the basal segments of the antennules. 

 The greater part of the ophthalmic peduncle is in this way con- 

 cealed in a kind of sheath, and only the terminal segment appears 

 and is movable within the orbit. 



In the Dromiacea the second segment of the antennal peduncle 

 is free and there is no corresponding process of the front, so that 

 the orbits are incompletely or not at all defined. The arrange- 

 ment is hardly more complete in certain Oxyrhyncha (Macrocheira) 

 (Fig. 147, B), but in most Brachyura the antennal peduncle joins 

 with the front to form a partition separating completely the 

 orbits from the "antennular fossae," into which the antennules 



17 



