84 
ZOOLOGICAL EXERCISES. 
mass round the base of one of the legs; notice the 
two large longitudinal muscles, one on each side. 
The anterior one is the extensor of the leg, the pos- 
terior is the flexor. Cut a portion of one out, mount 
in water, cover lightly with thin glass, and examine 
with the quarter objective. The muscular fibres are 
transversely striated, but not so distinctly as the 
muscles of the head. Now press down the cover- 
glass firmly and examine again ; nearly all trace of 
the striations will have disappeared. Cut off one of 
the legs and hold up the foot to the light ; notice the 
tendon of the flexor muscle going to the claws. 
9. Examine a portion of the corpus adiposum. 
10. Masticatory apparatus. Remove the animal from 
the water; dry it with a cloth, and fasten it down 
again to the cork with its ventral side upward. Dry 
the mouth with a piece of blotting paper. Remove 
with the forceps the different parts of the mouth, 
beginning with the labium, and arrange them in 
order on a glass slide. Notice the labiwm, divided 
transversely into two portions; the basal is dark 
coloured, and bears the three-jointed, black, labial 
palpi; the upper portion is light colored, and bears at 
its end four pale yellow, labial lobes, of which the 
outer are much the larger. The mawille ; small, two- 
lobed, the inner of which is toothed; the maxillary 
palpi are green, and five-jointed. The mandibles ; 
strong and horny, with denticulate, dark coloured 
tips. The labrum; broader than long, and deeply 
notched in front. 
11. The dorsal vessel.—Take another animal, kill it, and 
pin it down with the ventral side upward. Make a 
longitudinal incision up the abdomen and thorax, 
