456 Report of the State Geologist. 



second and third sets of muscles, called by Farre the oper- 

 cular muscjes, by Hancock the tube retractors, and by Allman 

 the antero-parietal vaginal muscles. They have their origin 

 near the top part of the ectocyst, and are inserted in the 

 flexible part of the invaginated tunic (endocyst) on which they 

 act. They consist of four flat bundles of stout linear fibers 

 (Plate A, i). The fibbers of the muscle are inserted one after 

 another in a straight line, commencing near the point of invagi- 

 nation, and extending some distance down the invaginated tunic. 

 The arragement of the fascicles causes the margins of the orifice 

 to fold in four portions in the retraction of its tube. The end is 

 consequently square, the angles corresponding to the insertion of 

 the muscles. The third set of muscles, the superior tube retractors, 

 consist of only four fibers. They have their origin below the other 

 tube retractors, and are inserted in the tegumental tunic above 

 them. The action of this set of muscles completes the action of the 

 second set. In the marine forms as represented by Bowerbanhid 

 densa, the muscles are the same, except that they have a tri- 

 radiate arrangement^ and the orifice is puckered into three folds 

 instead of four. 



The fourth set of muscles is for the purpose of closing the 

 orifice and was discovered by Mr. Hancock. They consist of 

 two sets of muscles passing around the invaginated tunic. One 

 set is made up of several fibers passing around the tunic at the 

 insertion of the inferior tube retractors and is of considerable 

 width ; the other consists of only two or three fibers and passes 

 around the tube at the interior of the superior tube retractors. 

 This action is to close the orifice on the retraction of the animal, 

 and also to oppose the tube retractors, which would have a 

 tendency, by their divergence, to keep the orifice open. These 

 muscles exist both in fresh-water and marine forms. 



The fifth set of muscles is connected with the endocyst and is 

 called the parietal muscles. They are numerous and strong (Plate 

 A,.m). Their extremities are probably attached to the ectocyst. 

 They run transversely, each muscle embracing a fourth or a third 

 of the cell, and consist of three or four fibers each. Their pur- 

 pose is the contraction of the endocyst, assisting in the propulsion, 

 of the animal. 



The sixth set (the posterior parietal muscles) is for the purpose 

 of preventing the entire invagination of the tunic and tentacles^ 



