44 



gill-slit, on the dorsal side of which it rests. PI. XI, fig. 131 will shew, however, that the 

 contact between the two organs only concerns the middle of the ventral vvall of the collar-canal, 

 the inner part of the wall resting on that portion of the trunco-collar septum in which the 

 muscles of the metasome end, while the outer part is confluent with the external part of the 

 septum. This relation is further ilhistrated by PI. XII, figs. 156, 155. If a tracing be made of 

 fig. 155 and if it be superposed on fig. 156, it will be seen that the long a.xis of the collar- 

 canal crosses the gill-slit at right angles; and the consideration of the figures which have been 

 referred to thus suggests the idea that the wall of the gill-slit in fact forms a fulcrum on which 

 the collar-canal moves. If this conclusion be correct, it would appear that the contraction of the 

 muscles pas.sing from the stalk to the inner and anterior wall of the collar-canal must depre.ss 

 that end of the organ, and elevate its outer end. But the contraction of the longitudinal muscles 

 of the trunk presumably increases the fluid pressure of the contents of the third body-cavity. 

 This increase would probably react mainly on the outer end of the collar-canal, partly because 

 the area of the septum exposed to the pressure is here greater than at the internal end, and 

 partly because the anterior horns of the third body-cavity are for the most part filled with 

 muscle. The pressure-effect would thus act in the same direction as the direct effect of the 

 contraction, and the projecting ventral lip of the external aperture of the collar-canal would 

 move in a dorsal direction. A collar-canal in this condition is seen in PI. III, fig. 24, the 

 external opening being here nearly closed, instead of having the widely open condition shewn 

 in PI. XI, fig. 131. 



When the collar-canal has assumed the position indicated in fig. 24, it seems natural 

 to suppose that the contraction of the strong oral muscle which traverses the collar-cavity will 

 restore the collar-canal to its former position by elevating the inner end of the organ, since 

 the origin of those fibres is immediately adjacent to the insertion of the metasomatic muscles. 



I am inclined to believe that the external opening of the collar-canal can be closed, 

 partly by the contraction of the musculature of the metasome, and partly by the movements 

 of the operculum. Even when the free edge of that organ is directed forwards, the operculum 

 might still be effective in helping to close the collar-pore, as is indicated by fig. 24, where a 

 sharp bend of the operculum is almost in contact with the ventral lip of the collar-pore. If the 

 collar-pores were closed, the arms, tentacles and other parts of the collar would presumably be 

 in full working order, since their body-cavities would form a closed system, and contraction of 

 any of the muscles would be capable of producing an effect on the fluid pressure of the collar- 

 coelom. Should any sudden retraction of the arms be required this could be effected by the 

 contraction of their longitudinal muscles, the fluid meanwhile escaping through the widely open 

 collar-pores. 



The "problematical tissue" {x.) of the collar-pores offers more serious difticulties. It is 

 in the first place necessary to decide whether the tissue is muscular or is a modification of 

 the basement-membrane, of skeletal or elastic properties. The evidence of stains seems to be 

 unequivocal on this ])oint. The tissue stains like the muscles. The clearest evidence is afforded 

 by specimens stained with haematoxylin and Orange G; and in these, while the basement- 

 membrane has taken up the haematoxylin colour, the problematical tissue, like the muscles of 



