BRYOZOA, WITH DESCRIPTIONS OF NEW SPECIES. 383 
of the animal, being at the same time antagonistic to the opening 
tendency of the tube-retractors, which, diverging from their 
insertions, must, in some measure, bring the walls of the inverted 
tube asunder. These sphincters are not readily distinguished, but, 
having seen them in several individuals, I have not the slightest 
doubt of their existence. Deeming, however, that it would be 
satisfactory to see whether a similar apparatus, for closing the 
orifice, could be found in the marine species, [ 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 
numerous, 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 observeras 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- 
retractors is not readily observed ; but when the polype is exserted 
there can be no doubt of its existence. 
The fifth set of muscles, P1.V. figs. 1 & 5 ¢, A, isin 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 
species. ‘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 towards 
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 satisfactory conclusion respecting 
* In the species examined by Dr. Farre, and named by him Bowerbankia 
densa, the tube-retractors have a “ triradiate arrangement,’’ and, consequently, 
the orifice is puckered into three folds, when the polype 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 diverging line, and arch 
considerablv outwards at the tips. In Dr. Farre’s species they arch outwards 
immediately above the disc, and are very little recurved at the tips. It is 
therefore pretty evident that there are two species, and that B. densa should 
not be merged in B. imbricata, which is most probably the form that I have 
geen. 
c3 
