64 



LEECHES 



'.m. 



b.t 



cr. d.h.c. v.h.c.^-"-^- d.v.m.^''* l-m. 



Fig. 33. Diagram of muscles of body wall of Hirudinaria. 

 Note oblique muscles, a.r.o^ annular receptor organ; b.t^ 

 botryoidal tissue; cm, circular muscles; cr, crop; d.h.c, dorsal 

 haemocoelomic channel; d.v.m. dorso-ventral muscles; ep, 

 epidermis; l.h.c, lateral haemocoelomic channel; /.m, longi- 

 tudinal muscles; o.m, oblique muscles; s.r.o, segmental receptor 

 organs; v.h.c, ventral haemocoelomic channel; v.m, vertical 

 muscles ; v.n.c, ventral nerve cord ; 1-5, serial numbers of annuli. 



FromBhatia, 1941. 



US some idea of what is likely to happen with fibres that are 

 contractile. 



Considering the nemertean as a fluid-filled cylinder in whose 

 walls runs a system of inextensible spiral fibres, we see that the 

 shape of the cylinder can be altered by changing the angle 6 which 

 the fibres make with the longitudinal axis (Fig. 34). The volume 

 contained by such a cylinder is maximal when the angle 6 is 54° 44'. 

 If the volume of the contained fluid were equal to the maximum 

 volume of the system there would be no possibility of changing 

 shape, so the volume of the fluid is something less than maximal 

 and the difference is taken up when necessary by the animal 

 becoming elliptical in cross section. In leeches the geometry is 

 comparable but the fibres are contractile. When the fibres are at 

 54° 44' to the longitudinal axis this is the position at which they 



