488 PORCELLANOUS AND VITREOUS FORAMINIFERA. 



medium, and contribute by their action to the nutrition of the 

 segment from which they proceed, these Pseudopodia are limited 

 in the latter case to the final segment, issuing forth only through 

 the aperture of the last chamber, so that all the nutrient mate- 

 rial which they draw-in must be first received into the last segment, 

 and be transmitted thence from one segment to another until it 

 reaches the earliest. With this difference in the Physiological con- 

 dition of the Animal of these two types, is usually associated a 

 further very important difference in the conformation of the Shell 

 — viz., that whilst the aperture of communication between the 

 chambers, and between the last chamber and the exterior, is 

 usually very small in the Vitreous Shells, serving merely to give 

 passage to a slender Stolon or thread of Sarcode from which the 

 succeeding segment may be budded-off, it is much wider in the 

 Porcellanous Shells, so as to give passage to a Stolon that may not 

 only bud-off new segments, but may serve as the medium for trans- 

 mitting nutrient material from the outer to the inner chambers. 

 There is no reason to believe, however, that anything like an Ali- 

 mentary Canal exists among Foraminifera ; the nutrition of the 

 entire body being doubtless effected by that interchange and circu- 

 lation of particles, which (as we have already seen, § 326) is con- 

 tinually going-on throughout its soft Sarcodic substance in this 

 form of the Ehizopod type. 



371. Between the highest types of the Porcellanous and the 

 Vitreous series respectively, which frequently bear a close resem- 

 blance to each other inform, there are certain other well-marked 

 differences in structure, which clearly indicate their essential dis- 

 similarity. Thus, for example, if we compare Orbitolites (Fig. 250) 

 with Gycloclypeus (Plate xvi., fig. 1), we recognize the same plan 

 of growth in each, the chamberlets being arranged in concen- 

 tric rings around the primordial chamber ; and to a superficial 

 observer there would appear little difference between them. But 

 a minuter examination shows that not only is the texture of the 

 Shell porcellanous and non-tubular in Orbitolites, whilst it is 



I vitreous and minutely-tubular in Cycloclypeus ; but that the par- 

 titions between the chamberlets are single in the former, whilst 

 they are double in the latter, each segment of the Sarcode-body 

 having its own proper shelly investment. Moreover, between 

 these double partitions an additional deposit of Calcareous sub- 

 stance is very commonly found, constituting what may be termed 

 the ' Intermediate ' or Supplemental Skeleton ; and this is 

 traversed by a peculiar system of inosculating Canals, which pass 

 around the chamberlets in interspaces left between the two laminae 

 of their partitions, and which seem to convey through its substance 

 extensions of the Sarcode-body whose segments occupy the cham- 

 berlets. We occasionally find this * Intermediate Skeleton ' extend- 

 ing itself into peculiar outgrowths, which have no direct relation 

 to the chambered Shell ; of this we have a very curious example 



