Many secrete a stalk at the hinder end: by "continuous" formation of this, without rupture at fission, a branching colony is formed (_Polyoeca_). This stalk may have a varying consistency. In _Anthophysa_ (Fig. 37, 13) it appears to be due to the welding of excrementitious particles voided at the hinder end of the body with a gelatinous excretion; but the division of the stalk is here occasional or intermittent, so that the cells are found in tufts at the apex of the branches. A corresponding secretion, gelatinous or chitinous, around the body of the cell forms a cup or "theca," within which the cell lies quite free or sticking to it by its surface, or attached to it by a rigid or contractile thread. The theca, again, may assume the form of a mere gelatinous mass in which the cell-bodies may be completely plunged, so that only the flagella protrude, as in Volvocidae, _Proterospongia_ (Fig. 75, p. 182), and _Rhipidodendron_ (Fig. 37, 14). Often this jelly assumes the form of a fan (_Phalansterium_), the branching tubes of which it is composed lying for some way alongside, and ultimately diverging. In _Hydrurus_, the branching jelly assumes the form of a branching Confervoid.[117] The cell-body may be bounded by an ill-defined plasmatic layer in Chrysomonadaceae and some Protomastigaceae,[118] or it may form a plasmatic membrane or "pellicle," sometimes very firm and tough, or striated as in Euglenaceae, or it may have a separate "cuticle" (in the holophytic species formed of cellulose), or even a bivalve or multivalve shell of distinct plates, hinged or overlapping (_Cryptoglena_, _Phacotus_, Dinoflagellates). The wall of the {114}Coccolithophoridae, a family of Chrysomonadaceae, is strengthened by embedded calcareous spicules ("coccoliths," "cyatholiths," "rhabdoliths"), which in the most complex forms (cyatholiths) are like a shirt-stud, traversed by a tube passing through the stem and opening at both ends. These organisms[119] constitute a large proportion of the plankton; the spicules isolated, or in their original state of aggregation ("coccospheres," "rhabdospheres"), enter largely into the composition of deep-sea calcareous oozes. They occur fossil from Cambrian times (Potsdam sandstone of Michigan and Canada), and are in some strata extremely abundant, 800,000 occurring to the mm. cube in an Eocene marl. The Silicoflagellates have siliceous skeletons resembling that of many _Radiolaria_, to which they were referred until the living organism was described (see pp. 79, 86 f.).