MOUTH- PARTS OF THE PAL/EMONID PRAWNS. 



51 



exopodite, there may be found in its proximal half a double 

 segment bearing two endites. At first sight, the endites upon 

 this segment appear to be the second and third, but the length 

 of the segment which precedes it, and the position of the endite 

 upon the latter at its distal end, suggest at once that that is 

 also a double segment, that here, as in Carides, the first endite 

 has been lost, and that the second double segment therefore bears 

 the third and fourth endites and is the basipodite. This 

 impression is strengthened by the fact that beyond the segment 

 with two endites there are five simple segments, as there are 

 beyond the endites which represent the basipodite of Cerataspis 

 (text-fig. 15), which, since in that respect it agrees with the 

 thoracic limbs of Malacostraca, may be taken as indicating the 

 original number of segments in this region in the malacostracan 

 maxilla. In the maxillule of Calanus (text-fig. 20) the first 

 endite is carried upon a fairly distinct segment, but the second, 

 third, and fourth lie upon a region which is undivided save by 

 an incomplete constriction, which marks off the distal portion of 



it, where the fourth endite and the exopodite are borne. The 

 maxilliped of Calanus (text-fig- 11) has a very short basal 

 segment bearing the small first endite, a long second segment 

 bearing the second, third, and fourth endites, and six distal 

 segments, which is one more than might be expected. Meristic 

 rearrangement has probably been at work in this portion of the 

 limb*. The maxillule of Ci/pris (text-fig. 21), which curiously 

 simulates the maxilla of Carides, seems, like the latter appendage, 

 to have lost the first endite. If that be the case, the exopodite 

 here stands opposite the third and fourth endites. The thoracic 

 limbs of Nebalia (text-figs. 8, 9), though they present no endites, 



* An increase beyond the normal of the number of joints in a crustacean limb 

 may take place in three ways : (1) by the annulation of the unsegmented apex of 

 either branch of the limb to form a "nagellum," as in antenme, thoracic exopodites, 

 and pleopods of Malacostraca, etc. Three faint annulations upon the apical 

 segment of the maxilla of Calanus are probably of this nature ; (2) by subdivision 

 of a segment, as in the carpopodite of various Carides ; (3) by resegmentation of a 

 limb or of part of it, as probably in the maxilliped of Calanus. 



