156 



of the flagellum are " auditory cilia," one or two on each joint. 

 These are very small but are of the usual shape being similar to 

 those of Asellus aquaticus as figured by Sars.* 



Loicer antenna. (Plate xxiii., fig, 3.) — The peduncle consists of 

 five joints and is considerably shorter than the llagellum. The 

 first joint is short, the second, third, and fourth, are subequal in 

 length, the fifth about half as long again as the fourth, but nar- 

 rower ; the third is rounded above and bears two small tufts of 

 setEe, small tufts are also present on the fourth and fifth joints. 

 The first joint of tlie flagellum is longer than the succeeding, the 

 next three or four -are usually short, the others subequal in length 

 but gradually becoming narrower ; towards the end they also 

 increase a little in length. Each joint bears one or two small 

 setae above and below at the distal end. 



The upper lip (Plate xxiii., fig. 4) is large and strong, regularly 

 rounded distally, the centre being slightly produced. It bears a 

 number of short setaj thickly set together and converging towards 

 the centre of the distal margin. 



The mandibles (Plate xxiii., figs. 5 and 5a) are large and powerful. 

 In a side view of the head the basal joint can be easily seen ex- 

 tending along the anterior portion of the lower margin of the head; 

 just below the base of the lower antenna it gives ofi' the three- 

 jointed palp which extends anteriorly beneath the antennae, from 

 this point the basal portion of the mandibles extends downwards 

 and forwards and curves inwards to form the cutting edge. From 

 the inner surface arises the large and powerful molar tubercle 

 which extends obliquely upwards and inwards almost at right 

 angles to the portion of the mandible from which it springs, until 

 it reaches the median line and meets the molar tubercle of the 

 other side. The two mandibles are of the same size and general 

 appearance but differ in a few details. It will be convenient to 

 describe the left mandible first. 



The cutting edge of the left mandible consists of two separate 

 processes one inside the other ; the outer one consists of four 

 sharp strong teeth, brown in colour, and the inner one of three 

 similar teeth. Figure 5a of plate xxiii., shows the ends of these 

 two processes as seen from the inside. Within these two processes 

 is another, the chitinous integument of which is less strong and 

 thickened ; it is rather slender but expands somewhat distally 

 and extends inwards to the median line so that the end of it is in 

 a line with the cutting edge of the mandible and the end of tlie 

 molar tubercle; the end is crowned with about 15 to 20 sharp 

 strong setse which project radially from the end. Between the 

 base of this process and that of the molar tubercle are four or 

 five plumose or pectinated setffi nearly as long as the last mentioned 



* Crustacea d'eau douce de Norvege, plate viii., fig. 19. 



