THE BESPIBATORY OBGANS. 71 



The five anterior abdominal rings each consists of a narrow 

 ventral plate loosely united by membrane to a dorsal arch, 

 which encircles more than four-fifths of the abdomen, and 

 which is perforated beneath by a pair of small round spiracles. 



The first abdominal segment is reduced above and laterally to an 

 exceedingly narrow ring; its ventral plate is well marked but very 

 short, although it is wider than the other ventral plates. The next 

 four abdominal segments enclose the greater portion of the abdominal 

 cavity. The first ring is contracted in front, the second is equally 

 contracted both in front and behind, whilst the two posterior rings 

 form the conical apex of the abdomen. The ventral plates of these 

 segments are longest from before backward ; and the last in the male is 

 deeply divided behind. The membrane between the ventral and dorsal 

 plates is sufficiently loose to allow of considerable extension of the abdomen 

 by the separation of the plates, but they are united by a series of flat 

 muscular bands which cover the whole inner surface of the abdomen. 

 These may be divided into three sets ventral, dorsal, and transverse. 

 The ventral muscles are the largest and most conspicuous. They form 

 pyramidal bundles on each side of the mesial line. They arise from the 

 anterior part of each ventral plate near the mesial line, and are inserted 

 into the anterior border of the next ventral plate, by an insertion rather 

 broader than their origin. A few fibres pass occasionally from one 

 muscle to join the insertion of the next. The transverse muscles are narrow 

 muscular bands, which arise from the sides of the ventral plates at 

 the junction of their middle and outer thirds, and are inserted into the 

 adjacent edges of the dorsal arches. The dorsal muscles arise from the 

 anterior part of each dorsal arch below, but near their posterior edges 

 above, and are inserted into the anterior edge of the next dorsal arch. 

 All the muscles are best seen by hardening the part in spirit or chromic 

 acid. This renders them opaque, so that they may be easily observed by 

 viewing the abdominal integument by reflected light. 



Section IX. The Respiratory Organ*. 

 PLATE I., PLATE V., Fias 6 AND 15 17, AND PLATE IX., Fia. 4. 



The spiracles or external openings of the respiratory organs 

 are ten in number on each side of the male, and eight in the 

 female. 



