. The Protozoa. Protozoa. THE SARCODINA 77 follows the outlines of the vesicles which form this zone of proto- plasm, the resulting skeleton forms a reticulum. Growth may take place more rapidly, however, at certain places, and spines, spicules, or protuberances of one kind or another are the result. The usual form of the network upon which the skeleton is deposited is an hexagonal mesh, but this may become modified in numerous ways, the apertures becoming either circular, polygonal, or elliptical (Fig. 41). When spines are formed, a secondary calymma may also be developed, carrying with it th
. The Protozoa. Protozoa. THE SARCODINA 77 follows the outlines of the vesicles which form this zone of proto- plasm, the resulting skeleton forms a reticulum. Growth may take place more rapidly, however, at certain places, and spines, spicules, or protuberances of one kind or another are the result. The usual form of the network upon which the skeleton is deposited is an hexagonal mesh, but this may become modified in numerous ways, the apertures becoming either circular, polygonal, or elliptical (Fig. 41). When spines are formed, a secondary calymma may also be developed, carrying with it the sarcodictyum, and the latter, in turn, may give rise to a secondary skeleton outside of the first. This process may be repeated until there are as many as six or seven acces- sory Fig. 41. — Schematic figure illustrating the modifications of skeletons according to mechanical principles of deposition. [DREYER.] The secretion is supposed to collect in the interstices between alveoli as at (c), forming simple spicules, or tri- and tetra-radiate spicules (b). Collecting in the lines of union of six alveoli, the deposit takes the form of an hexagonal mesh (d), which, by the addition of more material, becomes changed as at (a), (e), (/), and (g). A very interesting set of phenomena are connected with the acanthin skeletons where the spicules are not deposited in the calymma, but are formed at the centre of the central capsule, growing out centrifugally into the extra-capsular plasm and resulting in a skeleton of radiating spines. With a few exceptions these spines are twenty in number, and are arranged in a certain geometrical order which has been characterized as the Miillerian law. The points of the spines fall in five circles parallel to the equator, and there are four spines to each circle. The spines are named, according to this scheme, polar, tropical, equatorial, sub-tropical, and sub-polar (Fig. 42). The form of the silicious skeleton is quite varied. In its l
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