TYPE CRUSTACEA. 371 



are modified to serve as sensory organs and organs of masti- 

 cation. The first pair, termed the first antennw or antennules, 

 are usually sensory in function, though occasionally also loco- 

 motor (Ostracoda and some Copepoda), and are frequently 

 supplied with peculiar seta3 supposed to be olfactory in addi- 

 tion to others probably tactile in function (Fig. 163, B). They 

 consist in their typical form of a basal portion composed of 

 three or four joints, the terminal one bearing one (Fig. 163, B) 

 or two (Fig. 163, A} many-jointed flagella. The second pair, the 

 second antenna 3 termed also simply antenna? (Fig, 163, C and D], 

 are also principally sensory and consist typically of a two-joint- 

 ed basal portion, bearing two many-jointed branches. One of 

 these, that upon the outer side, frequently becomes reduced to 

 a scalelike rudiment (Fig. 163, C), the inner branch persisting as 

 the flagellum. The third pair, the mandibles, serve as mas- 

 ticatory organs and are generally much modified in correspon- 

 dence with this function. Typically (Fig. 164, A] they consist 

 of a two-jointed basal portion bearing two branches. The 

 proximal joint of the basal portion, however, becomes much 

 indurated by the thickening of the chitinous cuticle and also 

 toothed, forming the mandible proper, while the remaining joint 

 and the two branches undergo reduction even to disappear- 

 ance, being known when present as the mandibular palps 

 (Fig. 164, C, nip). The fourth pair are the first maxillce (Fig. 

 164, B and U) and serve like the mandibles for mastication, un- 

 dergoing a somewhat similar modification. They do not, how- 

 ever, become so indurated, though one or both of the basal 

 joints may be provided with stiff setae and serve as a jaw, and 

 the two branches more frequently persist than in the mandi- 

 bles. The fifth pair, the second maxilla', are also masticatory 

 and resemble the first in the modifications which they 

 undergo. 



The thoracic and abdominal appendages in all but the 

 lowest forms can be reduced to a typical appendage consisting 

 of a two-jointed basal portion tipped by two branches also 

 jointed. In appendages employed for swimming both 

 branches persist (Fig. 165, A), and may possess a broad 

 platelike form, but when modified for walking the outer 

 branch disappears. From limbs modified in this latter 



