116 CRUSTACEANS. 



well to remove that part of the carapace which covers the 

 right gill chamber at their bases. To do this, cut through 

 the carapace with fine-pointed scissors, along the groove 

 that marks the boundary between the branchial chamber 

 and the thorax proper. Begin at the posterior end of the 

 carapace, and cut forward along this groove to the cervi- 

 cal groove, and obliquely forward to its anterior end. 

 Turn down and break away the cover of the gill chamber, 

 and expose the gills. Observe their feathery appearance. 

 Observe that they fill the chamber, and come in contact 

 with all the water that passes through it. Move the legs 

 backward and forward, and note the effect on the gills. 

 Observe that the gills are arranged in vertical series cor- 

 responding in position with the legs. Observe that they 

 are also arranged in longitudinal series, the lower series 

 being attached to basal segments of the legs, and the 

 upper series (a double series) attached to a membrane 

 extending between the wall of the thorax and the legs. 



Examine with a lens, and compare in structure, the gills 

 of one complete vertical series. Tear a small gill to pieces 

 with needles, under water, to make out its structure. 



Remove now each leg, with its attached gill, proceeding 

 from the rear. To do this, place a knife blade between 

 the bases of the leg to be removed and the one in front of 

 it, and, by prying, break its hard attachments, and then, 

 with a sharp knife or scalpel, cut its membranous attach- 

 ments. Of the five pairs of legs, the first is used for 

 prehension and defense, and the others for walking. The 

 openings of ducts from the reproductive organs are little 

 pores perforating the inner angles of the basal segments 

 of a pair of legs, in males, the fifth or hindmost pair ; 

 in females, the third pair. 



Having removed the five legs of the right side, study 

 the foremost. Note its great size. Flex and extend it, 

 and study the structure and action of its joints, esDeciallv 



