392 THE CRAYFISH CHAP. 



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

 and segmented 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 b, the large 

 and complex ventral muscles, the lateral halves of which are 

 not separate from one another, the fibres being interwoven, 

 somewhat 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 appendages : 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 (p. 389), 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 

 fhe 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-cesophageal 

 ganglia ; b, the cesophageal connectives ; and c, the postoral 

 ventral nerve-cord, consisting of a large compound sub- 

 cesophagcal ganglion and of n 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 

 of feelers : the sub-oesophageal ganglion supplies the 



