CILIATA 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 fi 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 fi), 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, 1 have been described in the deepest layer 

 of the ectosarc in Heterotrichaceae. 



The innermost layer of the ectosarc is often channelled by a 

 system of canals, 2 usually inconspicuous, as they discharge con- 

 tinuously into the contractile vacuole ; but by inducing partial 

 asphyxia {e.g. by not renewing the limited supply of air dissolved 

 in the drop of water on the slide under the cover-glass), the 

 action of the vacuole is slackened, and these canals may be more 

 readily demonstrated. The vacuole, after disappearance, forms 

 anew either by the coalescence of minute formative vacuoles, or 

 by the enlargement of the severed end of the canal or canals. 

 The pore of discharge to the surface is visible in several species, 

 even in the intervals of contraction. 3 The pore is sometimes near 

 that of the anus, but is only associated with it in Peritrichaceae, 

 where it opens beside it into the vestibule or first part of the 

 long pharynx, often through a rounded reservoir (Fig. 60, r) or 

 elongated canal. 



The endosarc, in most Ciliates well differentiated from the 

 ectosarc, is very soft ; though it is not in constant rotation like 

 that of a Ehizopod, it is the seat of circulatory movements 

 alternating with long periods of rest. Thus it is that the food- 

 vacuoles, after describing a more or less erratic course, come to 

 discharge their undigested products at the one point, the anus. 



1 The " neurophane " fibrils of Neresheimer, Arch. Protist. ii. 1903, p. 305 f. 



2 Sometimes the number of afferent canals is limited to five {Paramecium), or 

 even one. There may be one or more contractile vacuoles, and in the latter case 

 the different ones have an independent rhythm. 



3 It is from such conclusive cases that the universal character of a discharge to 

 the surface has been inferred in the rest of Protista possessing this organ. 



