iv VERMES DERMO-MUSCULAR TUBE 195 



longitudinal musculature supplies the retractors of the proboscis (Fig. 

 138, p. 208); the number of these retractors varies, and is of great 

 importance for classification ; they are attached to the dermo-muscular 

 tube on the one hand at the anterior end of the proboscis (anterior 

 portion of the head), and on the other at the anterior or middle por- 

 tion of the trunk, and run freely through the body cavity. 



FIG. 128. Portion of the musculature of the body wall of Sipunculus, diagrammatic (after 

 Andreae). Im, Longitudinal muscles, partly left out ; rm, circular muscles ; dm, diagonal muscles, 

 cut away in the middle line ; bs, ventral chord ; rn, nerve rings. 



In the Phoronidce which live in chitinous tubes there is a typical 

 bilaminar dermo-muscular tube. 



In the Bryozoa we can no longer speak of a dermo-muscular tube. 

 Its extreme reduction is to be referred to the development of a stiff 

 skeleton (shells, cells, ectocysts), which deprives a dermo-muscular tube 

 of its function. Only such portions of the general musculature remain 

 as are necessary for the withdrawal and protrusion of the soft-skinned 

 anterior end with its tentacles, or (Ehabdopleura) for the contraction of 

 the stalk of the body, which is movable within its tube (longitudinal 

 muscles of the stalk). In the Endoproda also there is, especially in 

 the stalk, a delicate longitudinal muscular layer immediately under 

 the skin. The elastic cuticle (which takes the place of the circular 

 musculature), or (Pterobranchia) a cartilaginous substance which forms 

 an axial strand in the stalk, serves to counteract these muscles. 



The muscular apparatus which serves for the protrusion and withdrawal of the 

 anterior tentacle -bearing end of the body out of and into the cells, in those Bryozoa 

 provided with a temporary or permanent tentacle sheath, has been best observed in 

 the fresh-water forms (Fig. 139, p. 208). It consists esentially of 3 parts : (1) of 

 retractors which run in a longitudinal direction (like those of the Sipunculacca) 

 through the body cavity, and are attached on one hand to the anterior end of 

 the body near the tentacles, on the other to the body wall at the base of the cell ; 



