Systematic Position and Structure 17 



difference as " tube twice as long as wide " as against 

 "tube three times as long as wide," the distinction 

 is far too small to be certainly accurate ; besides, 

 there are always better characters, so that this never 

 need be used as a primary one, unless measurements 

 varying as much as " one by two " as against " one 

 by six," or something like this can be taken. Of 

 course there are some species whose breathing tubes 

 are so distinctive in shape that they can be recog- 

 nised at a glance, but the number and position of the 

 tufts and hairs on the tube and the situation of 

 the teeth in the pecten — a double row of spines on 

 the lower part of the tube, — sometimes also the shape 

 and number of the pecten teeth, are a far more re- 

 liable index. In the tube can be seen two large 

 tracheae, opening a short distance below the tip, 

 which is closed by a sort of chitinous valve. The 

 tube is of heavy chitin, like the head, and not very 

 movable, although it may be brought up at right 

 angles, or slanted well toward the tail. It is gener- 

 ally held almost perpendicularly to the eighth seg- 

 ment, with a slight backward inclination. 



The ninth segment is the smallest. It is very 

 movable, is generally saddled and frequently ringed 

 with chitin, and bears a set of four tracheal anal 

 gills. These are in many species distinctive in ap- 

 pearance, although their length is usually somewhat 

 variable within specific limits. They may be short 

 and bud-like, or longer than the ninth segment, and 

 even over twice as long as the breathing tube. There 

 are generally at least rudiments of tracheae to be 

 found in the gills ; sometimes the tracheae are large 



