GENUS POLYSTOMELLA. 2S5 



480. The meridional canals are further connected with the older and more internal por- 

 tions of the organism, as well as with the newer and more superticial ; this connexion being 

 established by a series of branches that pass between the two layers of septa in a radial 

 direction (as is shown on the septum If, fig. 1), from the meridional canals of each convolution 

 to the stolons which unite the segments of that convolution. These, which may be distinguished 

 as the converging branches, are, liowever, much less regular in their distribution than 

 those which pass outwards from the meridional canals to the stolons of the succeeding 

 whorl. 



481. Thus, then, it becomes apparent that by the two spiral canals, the number of con- 

 volutions of which equals that of the whorls of the shell, — by the very numerous meridional 

 canah, of which there is one for every segment of each whorl, — by the vast multiplication of 

 pairs of diverging brandies, of which each meridional canal sends off a number equal to that of 

 the connecting stolons between the segments, — and by the very considerable aggregation of con- 

 verging branches, which probably do not fall far short of the preceding, except in being single 

 while the}' are in pairs, — a very complete system of intercommunications is maintained between 

 the external surface and even the innermost portions of the shell. That these passages are 

 occupied in the living animal by prolongations of the sarcode-body, there can scarcely, I 

 think, be any reasonable doubt; and when we look to the remarkable development of what has 

 been elsewhere termed the " intermediate skeleton," but which may here be more appropriately 

 termed the " supplemental skeleton,"— namely, the secondary calcareous deposit which not 

 only forms the solid nucleus, but spreads itself over the entire surface, adding considerably 

 to the thickness of the spiral lamina, — it cannot be deemed improbable that the special pur- 

 pose of the canal-system is the formation and nutrition of this supplemental skeleton, which 

 has obviously no direct relation to the segments of the animal body contained within the 

 chambers. Through the trumpet-shaped diverging branches which open in such numbers 

 upon the surface of those chambers, and the straight canals which arise from the nucleus, 

 there will be abundant opportunity for the sarcode-body to extend itself over the whole 

 exterior of the shell, and thus to form any additional deposit upon its surface. 



482. The two forms of Polgstomella now^ described differ from each other simply in 

 those particulars wliich mark degree of development, and which the experience of similar 

 diversities elsewhere forbids us to account as of specific value, the general plan of structure 

 being essentially the same in both. The excess of size and turgidity which distinguish 

 P. craticiilata, as compared with P. crispa, are not more remarkable than the like excess by 

 which the large Dendritina of tropical seas is distinguished from the starved-out Peneroplis of 

 the Mediterranean ; and the extraordinary development of the supplemental skeleton with its 

 i-elated canal-system in P. craticduta loses its value as a differential character, when the series 

 of intermediate forms is traced out, in which the superadded parts are gradually reduced to the 

 rudimentary condition in which they present themselves in P. crispxt. In these forms (which 

 have been accounted distinct species by D'Orbigny and other systematists) we meet with 

 various kinds of surface-marking, which are related (like the diversities already described, H 473, 

 in P. craticiilata itself) to the varying amounts of calcareous deposit which have been added 

 to the exterior of the spiral lamina. Other species, again, have been founded on the extension 



