VI PRACTICAI. DIRECTIONS 377 



hand, "ami pull gently backwards and forwards (in the direction in 

 which the muscles pull). It will then be seen that the median and 

 lateral teeth come together in the middle line so as to act as a gastric 

 mill. Note the slit-like lurnen of the part of the gizzard behind this 

 and the arrangement of the se/.c which act as strainers. Make sketches 

 as you proceed. 



III. I. The chief muscles of the body are : — a, the paired and seg- 

 mented dorsal extensor, arising fro'm the side walls of the thorax, and 

 extending into the abdomen above the intestine, giving off slips to each 

 segment of the abdomen (this has already been removed) ; and b, the 

 large and complex ventral muscles, the lateral halves of which are not 

 separate from one another, the fibres lieing interwoven, somewhat like 

 those of a rope ; slips are given off to the abdominal sterna. These act 

 mainly as ^Jlexor of the abdomen. 



2. Muscles pass from the body to the proximal joints of the limb, 

 and between successive podomcres : — these latter will be examined at 

 a later stage (§ D). 



3. Note again the paired adductor of the mandible (p. 375), and 

 trace its calcified tendon downwards to its insertion on to the 

 mandible. <■ 



4. Tease out a small piece of muscle so as to separate its filires from 

 one another. Stain, and nioimt in glycerine. Note the transverse 

 slrialions, sarcoleinma, and nuclei (compare Fig. 32). Sketch. 



Remove the muscles of the liody described above, noting the sternal 

 artery (p. 375), and taking especial care to leave the abdominal nerve- 

 cord in situ when removing the large ventral muscles. Note that 

 in the thorax, the nerve-cord passes into a sternal canal, formed by a 

 series of ingrowths of the exoskeleton — the endophragnial system — from 

 which the muscles passing to the thoracic limbs arise. Insert the 

 scissors into the sternal canal, and cut away and remove its roof, bit by 

 bit. The whole of the central nervous system will then be exposed. 



IV. Observe that a more marked distinction into ganglia and 

 connectives is seen than in the case of the Earthworm, and that the 

 fusion of the two lateral halves of the cord or chain has only affected 

 the ganglia, the connectives being double all the way along. 



I. Note; — a. The brain, or' fused, com^o\ix\i supra-asophagcal ganglia; 

 h, the ccsophageal connectives ; and c, the postoral ventral nerve-cord, 

 consisting of a large compound snb-aspphageal ganglion and of 12 

 segmental ganglia, united by paired connectives- Beneath the cord, the 



