MUSEUM OF COMPARATIVE ZOOLOGY. 161 



numerous muscular elements, which are primarily arranged about the blood 



cavity. The relation of these to the body musculature is of considerable 



interest. 



c. Musculature. 



If a transverse section be made in the plane of the annular mass of 

 muscle surrounding the pharynx which is produced by the fusion of the 

 four retractors, tiiere appears only an iudetinite mass of confused hbres. 

 If, however, the section be cut in any longitudinal plane, it will be seen 

 that the longitudinal fibres which compose this mass divide into two 

 unequal parts, each of which draws its fibres from all parts of the origi- 

 nal muscular mass. In such sections each of these portions appears like 

 a band ; the smaller curves over into the muscularis of the body wall of 

 the introvert, or rather goes to form the longitudinal muscles of this, its 

 fibres being directly continuous with those of the predominant longitu- 

 dinal layer. The other and larger portion ascends into the tentacular 

 fold ; a few of its fibres follow the aboral surface of the blood cavity, but 

 by far the greater mmaber are continuous as an apparent muscular band 

 along the oral side of the cavity immediately adjacent to the latter. 

 At the base of the tentacular fold it is thickest, measuring half the 

 thickness of the oral wall in which it lies; but as it advances distad 

 through the tentacular fold, fibres are continually given ofi" peripherally, 

 so that they radiate toward the surface. These terminate in the vac- 

 uolated portion of the cutis in some manner not exactly determined. 

 In this way the muscular band becomes looser and looser by the gradual 

 loss of its elements, until at the tip of the tentacle only a few fibres 

 remain, which attach themselves there. In cross sections the tentacular 

 fold shows a few circular fibres, immediately adjacent to the blood cav- 

 ity, which turn into the trabeculse and cross into the corresponding layer 

 of the opposite wall of the fold. lu addition to these, the trabeculoe 

 have other fibres which cut the muscular band at right angles, and run 

 from one side of the tentacular fold to the other. At the outside of 

 the muscular band there can be found usually a few circular fibres. If 

 one considers that this muscular band, prominent in longitudinal sec- 

 tions through any plane, thus represents a muscular sheet extending 

 throughout the whole tentacular fold, and that this lies in the cutis 

 of the oral, or in an expanded condition convex, surface of the blood 

 cavity, with its fibres radiating into every fold of the tentacles, — if one 

 remembers, further, that it is at its base connected with the fused re- 

 tractors, and is in fact merely an extension of them, — then its action 

 in the inversion of the tentacles by drawing in and packing together 



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