186 Mr. A. Hancock on the Anatomy of the 



satisfactory to see whether a similar apparatus for closing the 

 orifice could be found in the marine species, I examined specimens 

 of Bowerbankia, and had the satisfaction of detecting sphincter 

 muscles in the same situations. At the point of insertion of the 

 inferior tube-retractors — according to Dr. Farre of the upper set 

 of opercular muscles — the circular fibres are very distinct and nu- 

 merous, forming a large portion of the inverted tube into a broad 

 sphincter. These fibres are so conspicuous that it seems strange 

 how they could have escaped the notice of so close and accurate 

 an observer as Dr. Farre. It is possible enough, however, that 

 they might be less developed in the species examined by him*. 

 The sphincter at the point of insertion of the superior tube-re- 

 tractors is not readily observed ; but when the polype is exserted 

 there can be no doubt of its existence. 



The fifth set of muscles, figs. 1 & 5 /, h, is in connexion with the 

 tunic or lining membrane of the cell, and is precisely similar to 

 the parietal muscles described by Dr. Farre in the marine spe- 

 cies. This set is formed of short, transverse belts, arranged in 

 pairs, considerably apart from each other, which are to be found 

 almost from end to end of the cell, but most conspicuously to- 

 wards the lower extremity. There appears to be two sets of these 

 fibres, one down the back, the other down the front of the cell ; 

 but I could not arrive at any very satisfactory conclusion respect- 

 ing their arrangement, neither could I determine their exact re- 

 lationship to the tunic, — whether they are attached to it by their 

 extremities only as supposed by Dr. Farre, or connected with it 

 throughout their entire length. Professor AUman appears to be 

 of the latter opinion, and certainly I saw nothing in confirmation 

 of that expressed by Dr. Farre ; though I am not sure that the 

 extremities are not attached to the cell-walls, thus giving to these 

 muscular belts fixed points of action. Howsoever this may be, 

 these parietal muscles undoubtedly have the power of contracting 

 the tunic, and so lessening the space within which the polype is 

 confined ; the contained fluid is made to press on the surface of 

 the polype, constraining it to pass upwards, and thus to effect its 



* In the species examined by Dr. Farre and named by him Bowerbankia 

 t/ew5a,the tube-retractors have a "triradiate arrangement," and consequently 

 the orifice is puckered into three folds when the ])olype is retracted. The 

 species referred to in the text we have seen has four such folds — the tube- 

 retractors being divided into as many bundles. The circle of tentacles also 

 assumes a different form in the two species : in that examined by me the 

 tentacles rise from the disc in a straight, slightly diverj^ing line, and arch 

 considerably outwards at the tips. In Dr. Farre's species they arch out- 

 wards immediately above the disc, and are very little recurved at the tips. 

 It is therefore pretty evident that there are two sj)ecies, and that B. densa 

 should not be merged in B. iinbricata, which is most probably the form that 

 I have seen. 



