^Structure of the Mouth in Sucking Crustacea. 257 



stitute a kind of semicylindrical tube, or short rostrum, placed 

 under the head and proceeding from the prosternum, formed 

 beneath by the closely joined maxillipeds, in front by the 

 clypeus and the upper lip, and of which the elliptic opening 

 is formed by the short terminal joints of the palpi of the 

 maxillipeds. This semicylinder hides all the other organs of the 

 mouth excepting the stipes and palpus of the mandibles. The 

 palpi of the mandibles are triarticulate, and easily discovered ; 

 when at rest they are bent inwards and upwards against each 

 other, so that their long, thin, pointed terminal joints appear 

 close together in the cleft between the basal joints of the 

 lower antennae ; a stronger magnifying power discloses that 

 the whole palpus is covered with sparse, short, fine hair, like 

 the palpus of the maxilliped, and that the elongate middle 

 joint has three long and thick setas on the under surface, 

 whilst the terminal joint at the apex carries four incurved 

 spines. But in order to find out the position of the stipes of 

 the mandible, the head must be examined from the side. It 

 is then observed that the lateral portions of the head, its 

 pleural, present a longitudinal ridge reaching from the base 

 to the fore end, passing just above the elliptic group of ocelli. 

 The fore end of the pleural forms a protuberance between 

 two deep sinuses — one above the ridge, for the first pair 

 of antennse, the other below the ridge, for the second pair 

 of antennse. The margin of the pleura is continued under 

 the head in a curve, past the articular sockets of the second 

 pair of antenna?, as far as a point opposite the apex of the 

 stipites of the maxillipeds, ending there in a sharp indentation 

 which opens outwards and upwards. The space bounded by 

 this curved pleural margin and the first joint of the palpus of 

 the maxilliped is occupied by the stipes of the mandible, which, 

 however, only towards the apex comes close to the palpus, a 

 space being left open at the base. The movement of the 

 mandible is governed by the indentation just mentioned, which 

 receives an articular protuberance of the mandibular stipes ; 

 but as the pleural margin overlaps a little the exterior margin 

 of the latter, the movement of the mandible is, at any rate, very 

 confined. 



In order to obtain a correct idea of the other parts of the 

 mouth, it is necessary to remove the maxillipeds, which hide 

 them from below. If we succeed in doing so without disturb- 

 ing the natural position of the organs, we observe the second 

 pair of maxillse with their straight inner margins closely joined 

 together, so as to form an open duct with carinate bottom, 

 somewhat contracted in the middle and with rounded apex. 

 Their two lobes are short, flat, coniform, furnished with sparse 



