182 Mr. A. Hancock on the Anatomy of the 



same horizontal plane, considerably apart from each other, and 

 attached by their outer extremities to the inner surface of the 

 tunic some way below the opening ; their inner extremities con- 

 verging towards the tentacular sheath are attached to it about 

 one-third from its superior termination. Plumatella has fifteen 

 or sixteen of these fibres, Fredericella about fourteen. Their ar- 

 rangement is perfectly symmetrical. They are for the purpose 

 of preventing the inversion of the whole of the tentacular sheath 

 on the protrusion of the polype ; and thus to confine the oral 

 extremity within a convenient distance above the mouth of the 

 cell. 



The true value of these muscles will be fully understood if we 

 refer to the marine genus Bowerbankia, in which they are defi- 

 cient, and of course the tentacular sheath can be completely in- 

 verted, and accordingly the animal is enabled to reach to a greater 

 distance than it could otherwise have done. But an apparatus 

 of extraordinary beauty is provided to obviate the inconvenience 

 that must have arisen from the great elevation of the tentacular 

 disc above the support of the horny cell. This is effected by 

 what may be considered an elongation upwards of the cell. 

 Numerous setse bound together by a membrane are attached to 

 the lips of the orifice, so that when the polype is exserted they 

 stand up in a circle surrounding the lower part of the exposed 

 portion of the animal and give support to it. By this means the 

 far-outstretched tentacular disc is brought completely under the 

 control of the muscles for directing its movements*. We thus 

 clearly see that this set of radiating muscles is a compensation 

 for the deficiency of the circle of setse in the freshwater polypes. 



The third set of muscles, figs. 1 & 2 n } i, consists of numerous, 

 separate, fine thread-like filaments placed considerably apart, 

 without order, but in the same radiating manner as those last 



* Dr. Farre has described this apparatus in his paper so frequently re- 

 ferred to, but seems scarcely to have arrived at a full knowledge of its func- 

 tion. He considers that it is " for allowing of the freest possible motion to 

 the upper part of the body in its expanded state, to which it affords at the 

 same time support and protection." On examining the animal in action it 

 is evident that the use of the apparatus is as I have pointed out. The circle 

 of setae is then seen to compress the lower portion of the extended polype ; 

 and when the tentacular disc moves from side to side the neck always bends 

 from the top of the setae at a decided angle, and does not gradually arch 

 away from the lips of the cell as might be expected were this contrivance 

 for the purpose of giving flexibility. The delicate membrane uniting the setae 

 is strengthened with numerous, minute transverse fibres, forming the whole 

 into a powerful sphincter, thus giving great firmness to the part. By this ar- 

 rangement Bowerbankia is enabled to raise the tentacular disc far above the 

 polype-cell, and yet to remain as perfectly under the control of the rota- 

 tory arid retractor muscles as is the tentacular disc of Fredericella and Plu- 

 matella, in both of which it is confined close to the orifice of the cell by the 

 action of the radiating muscular fibres. 



