388 



THE CRAYFISH 



CHAP. 



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

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



b, the anus, on the ventral side of the telson : Paired 



c, the auditory aperture, on the dorsal side of the basal joint 

 of the smaller feeler or antennule (it will be seen 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 gills 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 different 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 arthro- 

 branch and examine its structure, noting the afferent and 

 efferent blood-vessels in its stem. Sketch. 



