TIUARTHRUS BECKI 201 



may spring from what is here referred to the coxopodite, as 

 its basal portion is very broad and originates close to the 

 articulation of the protopodite with the coxal joint. The 

 proximal joint of the exopodite Q2~) is somewhat arched and 

 tapers rapidly. It extends to the ends of the pleurae, and is 

 the longest joint of either branch. The posterior edge is 

 finely denticulate, and carries a row of long setse. The 

 distal portion (T) is multiarticulate, being composed of ten 

 or more joints. In general form it is slightly crescentic, 

 with the margins thickened, the anterior one being strongly 

 crenulated. Long setse extend posteriorly from the crenu- 

 lations on the dorsal side of the leg, making a conspicuous 

 fringe along the distal half of the exopodite. 



Plate VI, figure 7, represents a dorsal view of Triarthrus 

 Bccki, showing the antennae and the exposed portions of the 

 appendages. The antennae and legs on the right side are 

 drawn from one specimen, and the legs on the left side are as 

 shown in another individual. The biramous character of the 

 entire series of thoracic legs is very evident, as is also the 

 distinction between the crawling and swimming members. 

 Figure 8 shows the right second and third legs of the free 

 thoracic segments. In figure 9 the upper exopodite is repre- 

 sented without setae, so as to bring out the structure in 

 greater detail. On the lower leg the setae are shown. 



The antennae are about as long as the head, and are com- 

 posed of short conical joints. They usually occur in the 

 position shown in figures 5 and 7, but occasionally lie close 

 to the margin, figures 3 and 4, and sometimes curve back- 

 ward over the head, as in figure 6. 



It is not necessary in this place to describe in detail the 

 development of Triarthrus Becki, but attention may be called 

 to two early larval forms. The youngest is shown on Plate 

 VI, figure 1, and may be compared with the first segmented 

 stage, figure 2, and with the adult, figure 7. At this early 

 stage the animal is less than one millimetre in length (.63 

 mm.), and has no distinct separation into parts. The divi- 

 sion into a cephalic and a caudal region is indicated by a 



