392 THE CRAYFISH CHAP. 



III. i. The chief muscles of the body: a, the paired and seg- 

 mented dorsal extensor, arising from the side walls of the thorax, and 

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

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

 by the large and complex ventral muscles, the lateral halves of which 

 are not separate from one another, the fibres being interwoven, some- 

 what like those of a rope ; slips are given off to the abdominal sterna. 

 These act mainly as a flexor of the abdomen (compare p. 370). 



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

 those between successive podomeres will be examined at a later 

 stage ( D). 



3. Note again the paired adductor of the mandible (p. 390), 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 fibres from 

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

 striations, sarcolemma, and nuclei (compare Fig. 32). Sketch. 



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

 artery (390), 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 endophragmal 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. 



1. Note: a. The brain, or compound supra-ccsophageal ganglia ; 

 b, the (Ksophageal connectives ; and c, the postoral ventral net ve-cord, 

 consisting of a large compound sub-cesophageal ganglion and of 1 1 

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

 ventral thoracic and abdominal arteries (p. 390) will be seen, the main 

 sternal artery passing between the connectives joining the fourth and 

 fifth postoral ganglia. 



2. The brain gives off nerves to the eyes and the two pairs ot 

 feelers : the subcesophageal ganglion supplies the mandibles and 

 five following pairs of appendages and their segments. Each of 



