206 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



ends of this, on the longitudinal commissures, are a pair 

 of small ganglionic enlargements, which give rise to a pair 

 of small nerves to the muscles of the mandibles, and also 

 to a small pair which run up onto the stomach, to the gas- 

 tric ganglia. 



Trace the longitudinal commissures backwards into the 

 sternal plastron, where they join the thoracic ganglia, a 

 white ring, perforated in the centre, and giving off, on 

 each side, nerves to the maxillae, the maxillipeds, and the 

 pereiopods. The sternal artery passes through the ring. 

 Carefully examine the ring with a lens, and notice that it 

 is made up of a number of small ganglia fused together, 

 but still showing traces of theii separate outlines. Notice 

 a small nerve which runs backwards from the ring into the 

 abdomen. 



z. The Auditory Organs. 



Cut away the external or lower surface of the large 

 basal joint of the antennule, and notice that it is almost 

 entirely filled by the auditory sac, an irregular, greatly 

 folded vesicle, the wall of which is chitinous, but somewhat 

 flexible. Notice that the wall of the sac is united to the 

 inner surface of the upper side of the shell which covers 

 the basal joint of the antennule. The sac has no external 

 opening, but a line, fringed with hairs, on the outer sur- 

 face of the joint, marks the line along which it is joined 

 to the shell. Cut the sac out, and opening, notice that it 

 does not contain any grains of sand or other solid bodies. 

 Examine its inner surface, under water, with a hand-lens, 

 or a low power of the microscope. Notice a row of long, 

 slender auditory hairs, which project from the wall into 

 the cavity of the sac. There are also great numbers of 

 much smaller hairs scattered irregularly over the inner 

 surface of the sac. 



