834 THE ANIMAL KINGDOM. 



eminently contractile. Distinct muscular fibrils are sometimes met with, and 

 have been best worked out in Stentor and Vorticellina. In the former they 

 are visible as fine longitudinal lines beneath the clear bands which bear 

 the cilia of the body : the peristome has a few circular and radial lines 

 beneath the adoral membranellae. In the Vorticellina the fibrils spread 

 from the apex of the body and are inserted into a thickened cuticular ring, 

 the spot where the posterior ciliary wreath developes when the animal 

 becomes free. Thence they extend to the edge of the peristome, which has 

 its own fibril-system. They are continued in the opposite direction down 

 the pedicle in Vorticella, Zootkamnium, and Carchesium as a spiral band. 

 In the last-named the band is interrupted at each bifurcation of the 

 pedicle. Trichocysts are lodged in the exoplasm of a few genera, for the 

 most part Holotrichan. They are minute, fusiform bodies : when dis- 

 charged they bear a fine thread attached to one end, or they are converted 

 into straight filaments, pointed at each end, one end however furnished 

 with a minute appendage 1 . 



There is a nucleus (endoplast) accompanied by one or more para- 

 nuclei (nucleoli s. endoplastules). It lies superficially as a rule, but is said 

 to rotate in some instances with the endoplasm. It may be round or 

 ovate, band-like, and then sometimes moniliform, i. e. beaded, the segments 

 being connected by a delicate thread. There is a nuclear membrane, and 

 contents variously disposed in different Infusoria. The paranuclei appear 

 to be diminutive nuclei, and either lie near the nucleus or are apposed 

 to it. Their number varies, indeed is inconstant in the same species. 

 There is always one, but there may be more, e.g. 3-4 in Spirochona 

 gemmipara, 28 in Stentor Roeselii. When the nucleus is beaded, a para- 

 nucleus may correspond to each segment, or the number of paranuclei 

 may exceed or fall short of that of the segments 2 . In shape a paranucleus 

 is globular ; it has a membrane. There are some interesting variations 

 from the typical structure. Loxodes rostrum and Holosticha Lacazei 

 (Hypotrichd) have a number of nuclei with paranuclei, apposed one to each 

 nucleus in the former, scattered in the latter. A few Holotricha e. g. some 

 species of Opalina, and Hypotricha e. g. some species of Holosticka, have 



Physiologic, xxxii. 1883 ; Ryder, Proc. U. S. National Museum, vii. 1884. Stokes states (Journal 

 New York Micr. Soc. i. 1885) that Leucophrys emarginata, though coloured by chlorophyl bodies 

 is exceedingly voracious. On the other hand, Gruber's Strombidium oculatum has no mouth (Nova 

 Acta, 46, p. 514). Stokes' Vorticella smaragdina is green throughout, but is perhaps not tinted 

 with chlorophyl (Amer. Naturalist, xix. p. 19). The marine Vorticellid Spastostyla (Rhabdo- 

 styla) Sertiilariarum harbours symbiotic Zooxanthellae (Entz, Mitth. Zool. Stat. Naples, v. p. 



. 



1 See Maupas, A. Z. Expt. (2), i. p. 611, and Entz, op. cit. supra, p. 383. It is generally as- 

 serted that the filament is coiled within a sac and everted, as in the nematocysts of e. g. Hydroids. 

 Greeff maintains that the freshwater Peritrichan, Epistylis flavicans , has Hydroid-like nematocysts, 

 but other observers have failed to find them. 



2 bee Maupas, A. Z. Expt. (2) i. pp. 527, 660-1. 



