4. MUSCULATURE AND PAPILLAE 49 



The scattered papillae of the nonmetameric part of the metasoma are 

 distinguished by their feebler development. Often they possess a more or less 

 extensive coelomic cavity, but this communicates broadly with the coelom of 

 the trunk. The large conical papillae in the zone of thickening have an especi- 

 ally well developed musculature in two parts — a circular musculature derived 

 from the longitudinal layer of the cutaneous muscle sac, and longitudinal 

 muscle fibres which play the role of retractors and are distinct from the 

 longitudinal muscle layer of the trunk (Fig. 35/4). Plaques, if they are de- 

 veloped on these papillae, are each secreted by one single large cell, or 

 occasionally by a very few such cells. It is interesting that the epidermal cells 

 surrounding the latter are exceptionally rich in tonofibrillae, which form a 

 strong bond between the edge of the plaque and the basement membrane 

 (Ivanov, 1959b). The plaque itself may be similar to that on the metameric 

 papillae or sharply distinguished from them by its hoop-like or rod-like shape. 



Finally, the metameric papillae of the postannular region, found only in 

 the Thecanephria, are notable for their inconsiderable size and weak de- 

 velopment. The small cavity of these papillae is filled up with longitudinal 

 and circular muscle fibres (Fig. 35B). The plaques here are transverse rods, 

 somewhat bent forwards (Ivanov, 1952). 



