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of one segment to the front edge of that of the next. In 

 the abdomen they occupy the greater part of the body, 

 and in the last three segments they are united to form 

 continuous bands, these segments being extended as one, 

 so that the creature is forced through the water with 

 great rapidity. 



(2) Each appendage possesses huge Hexor and 

 extensor muscles. The former are inserted into the side 

 of the body- wall a little behind the point of origin of the 

 leg, tlie latter a little in front of it, so that they pass 

 down into the leg somewhat obliquely. The joints of the 

 thoracic legs are connected with one another by an 

 elaborate system of Hexors and extensors, so that the 

 joints may be moved separately. The muscles of the 

 antennae are attached to the sides and roof of the cepha- 

 lothorax, so that these appendages can be moved in 

 any direction. The mouth parts are produced internally 

 into short calcified rods, which are attached laterally to 

 the floor of the cephalothorax by broad, strong bands of 

 muscle ; this ensures powerful leverage for the mastica- 

 tory apparatus. 



It will appear from the above that the muscles are 

 segmentally arranged, being broken up into successive 

 segments by the connective tissue which forms their 

 insertion into the body-wall. The muscle segments do 

 not correspond witli the external segmentation of the 

 body, their divisions falling in the middle of each seg- 

 ment of the exoskeleton. As in all the Arthropoda, the 

 muscle fibres are all striated, the striation often showing 

 with extreme clearness. Koehler (10) states that in 

 Isopods and Amphipods the usual relative positions of 

 contractile element and protoplasm are reversed, the con- 

 tractile substance being situated in the central region of 

 the muscular cell of the primitive fibre, and surrounded 



