GENUS HELICOSTOMA. 



501 



connecting the name of its discoverer with its future specific denomination. 

 Although, as already intimated, the contour and habits of the animalcule a<^ree 

 closely with those of Nassula and Hdicostoma, certain points have been recorded by 

 Mr. Carter concerning its developmental manifestations which invite special atten- 

 tion. According to this observer the matured individuals become encysted within 

 the internodes of semi-decayed Nitelte, and then split up into two, four, or eight 

 cleavage inasses, each of which is subsequently liberated from the cyst in a form 

 altogether identical with the parent Otostoma, but of smaller dimensions, and possess- 

 ing at this early stage a single contractile vesicle only. This process of multiplication 

 by encystment and segmentation, while of but rare occurrence among the more highly 

 organized Ciliata, commonly obtains among the Infusoria Flagellata, treated upon in 

 the preceding volume. Mr. Charles Stewart, of St. Thomas's Hospital, has recently 

 reported to the author the occurrence of an animalcule apparently indistinguishable 

 from Otostoma Carteri, from the neighbourhood of Plymouth, Devonshire. The 

 Sisyridion cochUostoma of Ernst Eberhard * is evidently closely allied to, if not 

 identical with, this same type. 



Genus VII. HELICOSTOMA, Cohn. 



Animalcules free-swimming, elastic, more or less ovate, finely ciliate 

 throughout ; oral aperture ventral, circular, conducting to a tubular pharynx, 

 which, after proceeding obliquely backwards to the centre of the body, 

 terminates in a helicoidal flexure. 



Excepting for the peculiar prolongation of the oesophagus with its terminal 



helicoidal flexure, this generic type corresponds closely with the genus Nassula, 



an intermediate form between the two being supplied by Mr. Carter's genus 

 Otostoma. 



Helicostoma oblonga, Cohn. Pl. XXVI. Fig. 54. 



Body oblong or almond-shaped, rounded posteriorly, pointed anteriorly, 

 usually somewhat flattened, three or four times as long as broad ; cuticular 

 surface finely striate longitudinally and transversely; cilia short and fine, 

 evenly distributed, sometimes vibrating irregularly, and presenting a tufted 

 aspect ; oral aperture situated at a distance of about one-fourth of the length 

 of the whole body from the anterior extremity, pharyngeal tube continued 

 obliquely backwards to the centre of the body, and there forming a loop- 

 like or helicoidal twist ; endoplast subcentral ; contractile vesicle postero- 

 terminal. Length 1-125" to 1-75". 



Hab. — Salt water. Movements swift, rotatory. 



This species, described, in company with many other interesting marine types, 

 by Dr. Ferdinand Cohn in the ' Zeitschrift fiir Wissenschaftliche Zoologie ' for the 

 year i865, is referred to by its discoverer as apparently coinciding to some 

 extent \vith the Lcucophra signata of O. F. Miiller, but of which the figure and 

 description given are not sufficiently explicit for actual identification. The transverse 

 striae of the cuticular surface, referred to in the foregoing diagnosis, are much less 

 conspicuously developed than those taking a longitudinal direction, and are to be 

 defined only with a careful adjustment of a high magnifying power, constituting in 

 this respect a test-object equal to that of the cross-striation of the diatom Plcurosigma 

 attenuatum. 



* ' Oster-Programm der Realschule zu Coburg,' 1862. 



