THE CRAYFISH 



7. Note the following apertures : Median a, the mouth, on the 

 ventral surface of the head, between the jaws ; /;, the anus, on the 

 ventral side of the telson : Paired <:, the auditory, aperttire, on the 

 dorsal side of the basal joint of the smaller feeler or antennule (it 

 will be seen better later on) ; d, the renal aperture -, on a conical ventral 

 elevation of the basal joint of the larger feeler (antenna) ; e, the genital 

 aperture, in the male on the basal joint of the last thoracic leg, and in 

 the female on the last thoracic leg but two. 



B. Respiratory organs. 



1. Carefully cut away the left gill-cover with scissors, and fix the 

 animal under water on its right side, so as to expose the left gill- 

 chamber containing the feathery-looking gills. The inner wall of the 

 chamber is formed by the proper wall of the thorax, and the chamber is 

 open behind and below. In front of the gills is a groove, in which a 

 flattened plate (see p. 367) works backwards and forwards during life, 

 driving the water out in front, and causing the bubbles already noticed. 



2. The ?//.$ are 18 in number, and each has the form of a bottle- 

 brush. The six podobranchs are external to the arthrobranchs and 

 pleurobranchs (p. 375), and each is attached to a large folded and 

 corrugated epipodite (p. 366). The gills are related to definite metameres, 

 as will be seen from the following table, in which ep stands for epipodite, 

 and v for the vestige of a gill. Note that the first pair of thoracic 

 limbs bears a simple large epipodite only. 



3. Turn down the podobranchs and make out the relations of the 

 arthrobranchs from the above table. Then turn these down also, or cut 

 them off, and note the single complete pleurobranch and the two 

 vestigial ones. Cut off an arthrobranch and examine its structure, noting 

 the afferent and efferent blood-vessels in its stem. Sketch. 



