GENUS POLYSTOMELLA. 283 



hended from an examination of the delineations of the internal casts given in Plate XVI, 

 figs. 7, 9 ; for the infiltrating substance which has penetrated the chambers has also found 

 its way not only into the main trunks, but also into the minute ramifications of this system, 

 and has thus given just that representation of their distribution and relations, which is 

 afforded in regard to the blood-vessels of the higher animals by a well-injected and clearly 

 dissected anatomical preparation. We observe, in the first place, that in each of what may 

 be termed the two polar regions of the spheroidal body, there is a continuous spiral canal 

 (fig. 9, e, c, e), which overlies the extremities of the segments. These two spiral canals com- 

 municate with each other by a very regularly disposed series of canals {dcV, dd'), which pass in a 

 meridittnal direction between the adjacent e.Kternal margins of the segments. And each of 

 these meridional canals gives off, in its course from one polar region to the other, a uniform 

 succession of pairs of short passages (/,/) that diverge from each other widely, one series 

 inclining backwards over the uniform anterior margin of the segment next behind it, whilst 

 the other series passes forwards in the intervals between the " retral processes" of the 

 segment next in front of it. The passages which thus diverge from the meridional canals of 

 the outer whorl speedily debouch at its surface ; but if we examine into the termination of 

 those appertaining to the inner whorls (which is best seen in such fragments as the one 

 represented in fig. 7), we find that they become continuous with the stolons of the whorl 

 which surrounds them, as is shown at c, c , fig. 9. Further, it may be perceived that each 

 of the meridional canals receives branches from the canal-system of the segment internal to 

 it J this point, however, can be more clearly made out in sections of the shell. 



478. The spiral canals are frequently brought into view for part of their course, by 

 sections passing through the shell in a direction parallel to the equatorial plane but at no 

 great distance from one of its lateral surfaces ; and if such a section passes through the plane 

 of the inner convolution of the spiral canal, it often shows that the spiral canal communicates 

 towards its centre, with an irregular set of lacmice, which are excavated in the deepest part 

 of the solid umbilical nucleus. If, again, the plane of such a section is nearer to the lateral 

 surface, the portion of it which passes through the solid calcareous nucleus is seen to be 

 perforated by numerous apertures disposed at pretty regular intervals, and corresponding to 

 the superficial punctations. The relation of these to the canal-system is clearly evidenced by 

 vertical sections, such as that represented in fig. 3 ; in which we see at e, e the orifices of the 

 spiral canals transversely or obliquely divided, whilst the solid calcareous nucleus (//, /'/') is 

 itself shown to be traversed by straight canals, which spring from the successive convolutions 

 of the spiral canal, and pass directly without branching or inosculation to the external 

 surface. That this remarkable portion of the canal-system does not fully show itself in the 

 "casts" represented in fig. 9, is easily understood, when it is remembered that the whole 

 substance traversed by the straight canals having been removed, their long and slender casts 

 would be left entirely without support ; and the points at which these have been broken oft" 

 from the cast of the spiral canal are in fact to be seen on a careful examination. 



479. It is shown by the comparison of vertical and horizontal sections of the shell with 

 fragments obtained by fracture (fig. 8), that tlie meridional canals {d d', d cl) are in reality 

 spaces left by the divergence of the two layers of which each septum is composed, in the 



