THE RAT-FLEA, CERATOPHYLLUS FASCIATUS BOSC. 443 



on the hone, and also to distinguish the flat-edged needle from 

 the pointed one. When dissecting I use the pointed needle in 

 my left hand for holding the object, and the flat-edged needle in 

 my right hand for cutting. The needles should be pushed far 

 into the wooden handle, so that only a short length is free, other- 

 wise the needle is too springy and is liable to snap under 

 pressure. 



The flea was left in the drop of salt-solution, where it is kicking 

 about violently and may succeed, if not watched, in getting out 

 on to the slide and hopping off*. It is therefore best to begin by 

 decapitating the flea. This can be done by holding it still with 

 the pointed needle and snipping off the head with the flat-edged 

 needle. The dissection can then be proceeded with in a manner 

 free from haste or anxiety. 



I will describe now a method of making permanent preparations 

 of the organs of the flea which I have found very useful. There 

 is not a single detail of anatomy described in this paper which I 

 could not demonstrate to a sceptic in my permanent preparations * 

 at a moment's notice. Let us take the abdominal nervous system, 

 for example. The complete nervous system of the flea consists, 

 as in other insects, of the three sets of nerve-ganglia: (1) the 

 cephalic ganglion -complex, or brain, situated in the head dorsal 

 to the digestive tract (supra-oesophageal) ; (2) three pairs of 

 thoracic ganglia, corresponding to the three thoracic segments : 

 (3) a chain of abdominal ganglia extending into the abdomen. 

 Parts (2) and (3) are ventral to the digestive tract and constitute 

 a continuous chain of pairs of ganglia, but the two ganglia of any 

 given pair are fused together so as to appear like a single ganglion- 

 mass. Each pair of ganglia is connected with the pair next 

 behind or in front by a pair of stout nerves, known as " connec- 

 tives," and it can be plainly seen that these connectives remain 

 distinct in each pair, and are not fused together like the ganglion- 

 pairs (PI. 26). The first pair of thoracic ganglia is connected 

 with the brain by a pair of peri-oesophageal connectives. From 

 the ganglia are given off* nerves to the various organs of the body. 



It is almost impossible to dissect out the brain, and to study its 



structure and relations sections would be necessary. It is difficult, 



but by no means impossible, to dissect out the thoracic ganglia. 



Major Christophers, I. M.S., who worked for a time in my 



* These preparations are now the property of the Club. 



