u6 



PROTOZOA. 



in fig. 26, a (Carpenter). Moreover, "in the higher types of the 

 hyaline or vitreous series we frequently meet with an ' intermediate 7 

 or ' supplemental ' skeleton, formed by a secondary or exogenous 

 deposit upon the outer walls of the chambers, by 

 which they receive a great accession of strength. 



This deposit not only fills up what would other- 

 wise be superficial hollows at the junctions of the 

 chambers (fig. 26, a d), or (as in Polystomelld) at 

 the umbilical depression, but often forms a layer 

 of considerable thickness over the whole surface, 

 thus separating each whorl from that which en- 

 closes it ; and it is sometimes prolonged into out- 

 growths that give a very peculiar variety to the 

 ordinary contour, as in some varieties of Rotalia 

 and Polystomella, but most characteristically in 

 Calcarina (fig. 26, b), and the stellate form of 

 Tinoporus. This intermediate or supplemental 

 skeleton, wherever developed to any considerable 

 extent, is traversed by a set of ' canals,' which 

 are usually arranged upon a systematic plan, and 

 are sometimes distributed with considerable min- 

 uteness " (Carpenter). The canals of this system 

 are doubtless filled in the living state by prolonga- 

 tions of the sarcode, which serve to keep up the 

 vitality of the intermediate skeleton. This intermediate skeleton, 

 with its canal-system, is largely developed in many of the highest 

 and largest of the types of the hyaline Foraminifera, and very 

 specially so in the " Nummulites " and their immediate allies. 



As regards the distribution of the Foraminifera in space, a few 

 forms (all belonging to the single family of the GromidcR) are fresh- 

 water in habit, but the vast majority are inhabitants of the sea. 



As regards the marine forms, very many species live in compara- 

 tively shallow water, but certain forms are found at great depths in 

 the ocean. A small number of forms, belonging, according to Dr 

 Henry B. Brady, to eight or nine genera, are pelagic in their habits, 

 and " pass their existence, either in part or entirely, at the surface of 

 the ocean or in mid-water." Though only a few genera have pelagic 

 representatives, these few are of great importance, owing to the ex- 

 traordinary profusion in which they occur individually. The princi- 

 pal pelagic genus is Globigerina, most of the species of which live in 

 the open sea, though one species of the genus seems to always live 

 at the bottom. The shells of the pelagic Foraminifera, after the 

 death of the animal, fall to the bottom of the sea, where they ac- 

 cumulate to form (along with the shells of the species which live 

 habitually at the bottom) great deposits of " Foraminiferal mud." 



Fig. 25. — Section of 

 the shell of Nodosaria 

 rapa, showing the 

 mode of formation of 

 the successive cham- 

 bers. Oral aperture 

 of the test ; w Wall ; z" 

 Primordial chamber. 



