{142}[Illustration: FIG. 50.—Ectosarc of Ciliata. _a-f_, from _Stentor coeruleus_; _g_, _Holophrya discolor_. _a_, Transverse section, showing cilia, pellicle, canals, and myonemes; _b_, surface view below pellicle, showing myonemes alternating with blue granular streaks; _c_, more superficial view, showing rows of cilia adjacent to myonemes; _d_, myoneme, highly magnified, showing longitudinal and transverse striation; _e_, two rows of cilia; _f_, _g_, optical sections of ectosarc, showing pellicle, alveolar layer (_a_), myonemes (_m_), and canals in ectosarc. (From Calkins, after Metschnikoff, Bütschli, and Johnson.)] The ECTOSARC has a very complex structure. Like other protoplasm it has a honeycombed or alveolate structure, but in this case the alveoli are permanent in their arrangement and position. Rows of these alveoli run under the surface; and the cilia are given off from their nodal points where the vertical walls of several unite, and wherein the basal granule or blepharoplast is contained. Longitudinal threads running along the inner walls of the alveoli of the superficial layer are differentiated into muscular fibrils or "myonemes," to which structures so many owe their marked longitudinal striation on the one hand, and their power of sudden contraction on the other. The appearance of transverse striation may be either due to transverse myonemes, or produced by the folds into which the contraction of longitudinal fibrils habitually wrinkles the pellicles, when it is fairly dense (Peritrichaceae); circular muscular fibrils, however, undoubtedly exist in the peristomial collar of this group. Embedded in the ectosarc are often found TRICHOCYSTS,[156] analogous {143}to the nematocysts of the Coelenterata (p. 247), and doubtless fulfilling a similar purpose, offensive and defensive. A trichocyst is an oblong sac (4 µ long in _Paramecium_) at right angles to the surface, which on irritation, chemical (by tannin, acids, etc.) or mechanical, emits or is converted into a thread several times the length of the cilia (33 µ), often barbed at the tip. In the predaceous Gymnostomaceae, such as _Didinium_, the trichocysts around (or even within) the mouth are of exceptional size, and are ejected to paralyse, and ultimately to kill, the active Infusoria on which they feed. In most of the Peritrichaceae they are, when present, limited to the rim around the peristome, while in the majority of species of Ciliata they have not been described. Fibrils, possibly nervous,[157] have been described in the deepest layer of the ectosarc in Heterotrichaceae.