52 



POLYPIFERA. 



portion of the tegumentary membrane, which, 

 by the molecular deposit of earthy matter 

 in its tissue, becomes ossified, something 

 like the cartilage of the higher animals, with- 

 out ceasing to be the seat of nutritive move- 

 ment. It is evident likewise that what is 

 usually called the body of the Bryozoon 

 constitutes, in fact, but a small portion of it, 

 principally consisting of the digestive appa- 

 ratus. 



As to the operculum, destined to close the 

 entrance of the tegumentary cell, it is merely 

 a lip-like fold of the skin, the marginal portion 

 of which acquires a dense consistency by in- 

 terstitial deposit, while at the point where it 

 is continuous with the general envelope it 

 remains sufficiently soft and flexible to form 

 a sort of hinge. 



The tegumentary sac, deprived of its car- 

 bonate of lime, seems to be formed of a to- 

 mentous membrane, covered, especially upon 

 its inner side, with a multitude of cylindrical 

 filaments, disposed perpendicularly to its sur- 

 face, and closely crowded together. It is in 

 the interstices left by these fibres that the 

 calcareous matter appears to be deposited; 

 for if a transverse section be examined with 

 the microscope the external wall is seen not 

 to be made up of superposed layers, but of 

 cylinders and irregular prisms arranged per- 

 pendicularly to the axis of the body. 



But the above are not the only arguments 

 adduced by Milne Edwards in proof that these 

 polyparies are maintained in vital connection 

 with the animal. On examining the cells at 

 different ages it is found that after they are 

 completely calcified they undergo material 

 changes of form. 



This examination is easily made, seeing 

 that in many species the young sprout from 

 the sides of those first formed, and do not 

 separate from their parents ; each skeleton, 

 therefore, presents a long series of generations 

 linked to each other, and in each portion of 

 the series the relative ages of the individuals are 

 indicated by the position which they occupy. 

 It is sufficient, therefore, to compare the cells 

 situated at the base, those of the middle por- 

 tion, those of the young branches, and those 

 placed at the very extremities of the latter. 

 When examined in this manner it is seen that 

 not only does the general configuration of the 

 cells change with age, but also that these 

 changes are principally produced upon the 

 external surface. For instance, in the young 

 cells of Eschara cervicornis, the subject of 

 these observations, the walls of which are of 

 a stony hardness, the external surface is much 

 inflated, so that the cells are very distinct 

 and the borders of their apertures prominent ; 

 but by the progress of age their appearance 

 changes, their free surface rises so as to extend 

 beyond the level of the borders of the cell, 

 and defaces the deep impressions which 

 marked their respective limits. It results that 

 the cells cease to be distinct, and the polypary 

 presents the appearance of a stony mass, in 

 which the apertures of the cells only are 

 visible. 



Fig. 59. 



Portion of a branch of the polypary of Eschara cer- 

 vicornis, magnified 20 diameters to show the form and 

 arrangement of cells. (After Milne Edwards.) 



Muscular system. The muscular system of 

 Bowerbankia is described as follows. 



For the process of retraction two distinct 

 sets of muscles are provided ; the one acting 

 upon the animal, the other upon the flexible 

 part of the cell. 



The muscles for the retraction of the ani- 

 mal are contained in the visceral cavity, and 

 consist of two bundles of delicate thread-like 

 chords (fig. 56, 8 and 9); the one set, 

 arising from the bottom of the cell, to be 

 inserted about the base of the stomach ; the 

 other, also arising from near the bottom of 

 the eel), though generally at the opposite 

 side from the former, and passing up free 

 by the side of the pharynx, to be inserted 

 around the line of junction of this organ 

 with the base of the tentacula. The muscles 

 provided for the retraction of the opercu- 

 lum, or flexible portion of the cell, have 

 their origin from the inner surface and near 

 the top of the stiff part, and are inserted into 

 the flexible portion on which they act. They 

 are most distinctly seen when the flexible 

 operculum is completely drawn in', at which 

 time the latter is folded up so as to occupy 

 the axis of the upper part of the cell, and to it 

 the muscles are seen extending from the oppo- 

 site sides of the cell from which they have 

 their origin. They consist of six flattened 

 bundles of fibres, having a triradiate arrange- 

 ment. The upper three sets (fig. 60, a, 3) 

 act upon the upper part of the cell, and are in- 

 serted into it. The lower three (fig. 60, a, 4) 

 are smaller, and are for the purpose of re- 

 tracting the bundle of setae with which it is 

 crowned. 



These fasciculi afforded Dr. Farre an ex- 

 cellent opportunity for investigating the struc- 

 ture of this form of muscle. It would appear 

 as if muscular fibre were reduced to its sim- 

 plest condition. The filaments are totally 

 disconnected, and are arranged the one above 

 the other in a single series. They pass straight 

 and parallel from their origin to their inser- 

 tion, and have a uniform diameter through 

 their whole course, except that each filament 

 generally presents a small knot upon its centre, 

 which is most apparent when in a state of 

 contraction, at which time the whole filament 

 also is obviously thicker than when relaxed. 



