392 University of California Publications in Zoology. [ VOL - 6 



longitudinal muscles of the body, most numerous toward the 

 anterior extremity and in large masses around the margin of 

 invagination ; on contraction of the inner longitudinal muscles, 

 the whole mass of the acetabulum is drawn back (pi. 40, fig. 43). 

 This is the state invariably found in killed specimens. In the 

 living animal in the expanded state the mouth of the acetabulum 

 is at the anterior extremity (pi. 33, figs. 7, 9) ; further evagina- 

 tion is impossible. The great lengthening of this region observed 

 in active specimens must be due to expansion of the acetabulum 

 itself. 



The fibres of the outermost layer (out. mer.), constituting 

 the bulk of the acetabulum, are derived from the inner longi- 

 tudinal layer of the body and are meridional in direction, pre- 

 senting in section a margin of cut edges surrounding the aceta- 

 bulum. This layer is densely supplied with finer radial fibres 

 (rad. fib.), at right angles to the long axis of the acetabulum, 

 passing from dorsal to ventral surface. These are the homo- 

 logues of the sagittal fibres of the body musculature, greatly 

 increased in size. 



Immediately within this outer heavy layer lies a set of fibres 

 predominantly transverse (out. arc.), passing from left to right 

 of the acetabulum, the homologue of the inner transverse layer 

 of the body. This layer is even more abundantly supplied with 

 radial fibres than is the outermost one. These pass from dorsal 

 to ventral, at right angles to the radial fibres of the outer layer. 

 Within this layer is a second set of meridional fibres (in. mer.) 

 the homologues of the outer longitudinal muscles of the body, 

 followed by a thin layer of fibres passing circularly around the 

 opening of the acetabulum, corresponding to the outer transverse 

 muscles of the body. Lying next to the cuticula are thin longi- 

 tudinal and transverse layers, the homologues of the cuticula r 

 musculature elsewhere. At the mouth of the acetabulum a 

 special sphincter muscle has been developed, composed of fibres 

 from the inner transverse, the inner longitudinal and the circular 

 muscle layers of the acetabulum (acet. sphinc.). Most of the 

 fibres are circular in direction ; they are grouped in heavy bundles 

 and form a large ring-muscle about the acetabular opening. 



The musculature of the funnel-region has been worked out 



