3 8 Practical Zoology. 



6. Study carefully the structure and action of the tail fin. Its 

 middle piece is the telson, underneath which is the external 

 opening of the intestine, .the anus. 



Remove the telson, and without disturbing the side parts of the 

 tail fin, separate the sixth abdominal ring from the fifth. Now 

 carefully compare this (sixth) ring and its appendages with the 

 third ring and its appendages. 



7. Are the appendages of the thorax borne upon rings like 

 those of the abdomen? If so, where are the rings? With forceps 

 seize the base of one of the hindmost pair of walking legs, and 

 move it backward and forward; are these borne on a distinct 

 ring? Carefully clean the sternum between the other walking 

 legs, and look closely for indications of rings. 



8. With the forceps break away one side of the carapace, 

 beginning at the lower edge. This lays bare the white, feathery 

 gills. Cover the specimen with water in the dissecting pan to 

 show the gills more clearly. Move the legs of this side back and 

 forth, watching the gills. 



9. Study now the hindmost of the walking, or thoracic, legs. 

 Count its segments. Observe how the first segment is joined to 

 the body. Flex the leg as far as possible, in every direction, 

 noting the number of joints, and the motions allowed by each. 

 With the forceps seize the squarish, basal segment of this leg, and 

 pull off the leg. 



10. Remove in like manner the legs in front of this, again 

 being careful to get a firm hold of the short, wide segment next to 

 the body. What is the relation between the leg and the gill 

 nearest to it? Lay this leg on a paper in front of the one previously 

 removed. In this way pull off all the legs of one side, from the hind- 

 most to the foremost, laying them in order. Compare them all 

 with the one first taken. In the legs bearing pinchers is there 

 any real new part added, or is the pinching apparatus produced by 

 some change in a part presented in all the legs? How do the 

 legs which bear the big claws differ from the walking legs? Com- 

 pare them, segment with segment. 



