PHYLUM ARTHROPODA 



517 



'Stl'jl 



third pair consists of two strong toothed bodies of a deep brown 

 colour, placed one on each side of the mouth, and called the man- 

 dibles (md.). The remaining appendages form two rows of delicate 

 leaf -like processes, attached to the segmented portions of the body, 

 and overlapping one another from before backwards : their number 

 varies from forty to nearly seventy (th.f., abd.f.). 



Appendages. The antenmtle (Fig. 430, 1) consists of a bent 

 rod bearing delicate chitinous bristles or setce at its tip, and pre- 

 senting, at the bend, a joint, due to the presence of an articular 

 membrane. The appendage is thus 

 made up of two podomeres or limb- 

 segments, movably articulated to- 

 gether. Its function is probably 

 tactile. 



The antenna (2) is absent in 

 some species both of Apus and 

 Lepidurus : in A. cancriformis it 

 is a very delicate hook-shaped 

 unjointed structure, probably 

 functionless. As we shall see from 

 the study of development, it is a 

 vestigial organ. 



The mandible (3) is also an un- 

 jointed appendage. It has the 

 form of a deeply concavo-convex 

 plate, strongly chitinised, and pro- 

 duced along its inner edge into 

 strong teeth. The mandibles lie 

 one on each side of the mouth, and 

 are so articulated that, by means 

 of muscles, their toothed edges can 

 be brought together in the middle 

 line, so as to rend the food. 



The fourth and fifth appendages 

 are very small, and are probably 

 functionless or nearly so : they 

 follow one another just behind the 

 mandible, and are called the first and second maxillce. The first 

 maxilla (4) consists of two curved chitinous plates, the second of a 

 basal portion produced into two branches (5). Between the first 

 maxilla and the mandibles are a pair of delicate unjointed pro- 

 cesses, the paragnatha (Fig. 428, pgn.) : they form together a sort 

 of lower lip, and are not usually reckoned as appendages. 



The foregoing appendages all spring from the unsegmented 

 anterior portion of the body or head. As we shall see, however, 

 the succeeding limbs arise each pair from its own segment, so 

 that the presence of five pairs of appendages on the head may 



Fio. 429. Apus glacialis, ventral 

 aspect, abd. f. abdominal feet ; antl. 

 antennule ; ant2. antenna ; Ibr. labrum ; 

 md. mandible ; mx. first maxilla ; ov. 

 aperture of oviduct ; s.f. pi. sub-frontal 

 plate ; sh. gl. shell-gland ; th.f. thoracic 

 feet ; th.f. 1, first thoracic foot. (After 

 Bernard.) 



