3G THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBAEUS 



Just dorsal to this row of long plumes there is a row of much fewer and re- 

 latively short acicular seta>, shown in hlaek in figure 43. In the reentrant 

 angle between the two lobes of the telson, each side, there is a rounded spine 

 similar to the one formed on the sixth pleopod (fig. 42) where the basal lobe 

 of the exopodite joins the distal lobe. 



Beyond this perfection of the locomotor apparatus of the alidomen the 

 third larva differs but slightly from the second though there are differences in 

 size, proportion, color and habits. Compai-ing figure 40 with figure 23 it is seen 

 that the abdomen is wider, the cephalothorax more cylindrical, the limbs stouter, 

 the entire aspect more heavy and crayfish-like. 



The third larva is still sufficiently transparent to allow the heart to be 

 seen through the shell, luider the microscope, beating at about three times a 

 second ; the stomach and intestines also show dimly as dark areas. 



The animal is not conspicuous upon sand or mud; its color is, as before, 

 light with fine, dark-red specks. The eyes are black and the chel« not red but 

 pink. The liver region is greenish but the dark yolk so long conspicuous has dis- 

 appeared with the perfection of sensory and locomotor oi-gans. Where 

 tlie lit'ail-tht)rax ends posteriorly there is a dark rim caused by concentration of 

 pigment cells there and the abdomen still has a dark ))and across the dorsal 

 side of the first somite. The region above the heart is quadrangular and very 

 pale in color. Back of each eye there is a dark longitudinal band. The groove 

 between the head and thorax dorsally is light and the pigment anterior to it 

 is more dense. Under the microscope the arborescent red pigment cells often 

 have a^ l)lue background and in some regions there are arborescent yellow cells 

 amongst the red ones. 



The considerable increase in size in passing from the second to the thii'd 

 stage may be seen by comparing the measurement given above, page 28, with 

 the following. In third stage a larva measured 14 nun. from rostrum tip to 

 edge of telson and 15 mm. to end of plumose seta*. The antenna was 12 mm. and 

 the chela 10 nun. long. The width of the thorax was 3.5 mm. and its depth 

 4 mm. The telson was 2 mm. wide and the fan formed by it and the sixth pair 

 of pleopods 5 mm. without the plumose set;B, and 6 nun. with them. The length 

 of the head-thorax was 7 mm. and of the abdomen 7 mm. 



In the third stage the larvae were active and voracious, walking and swim- 

 ming with ease and speed so that they were hard to catch. '\Anien kept to- 

 gether they soon lost chelfe in fight with one another and gi'eedily devoured their 

 dead fellows. When a piece of frog's muscle was put into a dish with these 

 larvae they seized as soon as they came into contact with it and holding the main 

 mass with their chelae and other claws dragged it backward while tearing off 

 fragments with their mouth appendages. Thus twenty-three to twenty-five 

 days after hatching larva^ in the third stage which had had no flesh food, unless 



