FROM THE SOCIETY S GARDENS. 



689 



establishes beyond doubt the interesting character of the basal 

 plate. The sole remaining ventral part of the genital chamber 

 becomes smaller and smaller until it is no more than a narrow- 

 cleft below the basal plate. ISText, its lower wall becomes chiti- 

 nous and is approximated to the basal plate, which is for the rest 

 of its course a circle of chitin squashed perfectly flat into a plate 

 with only a narrow rift between (text-fig. 31 ). 



The interest in this observation centres in the fact that it 

 explains the nature of the basal plate. At the base (near the 

 parameres) this begins as an ordinary squamiform apodeme on 

 the lower wall of the sac within the genital chamber. It runs 

 back (i. e. in the direction of the head) as an ordinary tubular 

 apodeme formed as an invagination of the ectoderm in the lower 

 part of the genital chamber. This " tube " is compressed into a 

 flat plate and its lumen reduced to a thin rift— continuous with 

 the genital chamber. 



Ttxt-fi£fure 34. 



Tiipeuriis forfictdatus, $ . Cross-section tlnougli the extruded sac just 

 beliind the opening. 



D. ductus. M. muscle mass. Fl. narrow cliitinous strip on ventral surface. 



Extrusion and Retraction. — On the ventral surface of the basal 

 plate there is a series of longitudinal muscles which arise in front 

 from the anterior portion of the basal plate and are inserted 

 behind into the terminal sternite of the abdomen, serving to 

 thrust the plate forward and expose the parameies thi'ough the 

 terminal abdominal opening. Text-fig. 28 a shows that the para- 

 meres along their dorsal margin curve \i\ somewhat. Undei'neath 

 this overhanging ledge small muscle-fibres run back along the 

 length of the parameres and are attached to the base of the basal 

 plate, doubtless serving to draw the tip of the fused parameres 

 upwards in a dorsal direction, which is its usual position when in 



