THE CRAYFISH 97 



(12) Remove in like manner all the thoracic legs being 

 careful not to leave any part behind on the body. How 

 many gills are present on each ? Are any gills left behind ? 

 Do you see indications of rings ? Lay the legs on a piece of 

 paper in the order that you pulled them out. Compare 

 them all with the first taken out. In the legs bearing claws 

 do you find any new part added, or is the pinching appara- 

 tus produced by a change in parts that are already present ? 

 How do the legs which bear the big claws differ from walk- 

 ing legs? Compare them segment with segment. Draw, 

 (XI). 



(13) In front of the big legs, or great chelae, are sev- 

 eral pairs of appendages surrounding the mouth. Probe 

 between them to find the mouth. The appendages in front 

 of the great chelae are the hindmost of three pairs of jaw 

 feet called maxillipeds. Gently raise them to see how they 

 cover the other mouth parts. Note that these maxillipeds 

 have an inner branch covering the corresponding part of 

 the other maxilliped and an outer one. Are these branches 

 attached to one another or to a basal segment? Seize the 

 lower part and remove the whole maxilliped. Compare it 

 with a swimmeret of the third abdominal segment. In the 

 same way remove the second and first maxillipeds keeping 

 them in order. Are gills attached to the maxillipeds? 

 Make drawing of gill showing structure. 



(14) Anterior to the maxillipeds are the two pairs of 

 maxillae. These are very thin and lie close to each other, 

 so that if great care be not taken they will be pulled off 

 together. Insert the forceps deep down and remove them 

 one at a time. Notice at the base of the first maxilla re- 



STU. IN zooL. — 7 



