586 ORDER HETEROTRICHA. 



the right-hand peristome-border ; an ovate endoplast usually present in 

 both the anterior and posterior body-half, frequently connected by a cord- 

 like funiculus or commissure ; colour deep purple-red. Length 1-96" to 

 1-72". HAB. Fresh water. 



This species is distinguished from the preceding type by the greater length of its 

 body, the smaller proportionate extent of the peristome field, and the more highly- 

 developed undulating membrane. Stein obtained it from the neighbourhood of 

 Prague, and identifies it (in pars) with the Blepharisma persicum of Eberhard, but 

 not with the animalcule bearing that name as recorded by Perty. 



GENUS III. SPIROSTOMUM, Ehrenberg. 



Animalcules free-swimming, highly elastic, contractile and changeable 

 in shape, usually of great length in proportion to their width, cylindrical 

 or slightly flattened, rounded in front, truncate posteriorly ; peristome- 

 field elongate, extending down the left-hand side of the ventral surface as 

 far or farther than the middle of the body, widest at this point and continued 

 obliquely inwards as a short funicular oesophagus ; adoral cilia bordering 

 the outer or left-hand side only of the peristomal area ; no undulating 

 membrane ; endoplast ovate or moniliform ; contractile vesicle canal-like, 

 usually with a large posterior dilatation ; anal aperture postero-terminal. 

 Inhabiting fresh water. 



Among the innumerable protean forms assumed by the representatives of this 

 genus, that associated with a spiral contraction upon their long axis is most frequent, 

 such contraction imparting to the peristome the appearance of being spirally con- 

 volute around the animalcule's body. In Pritchard's 'Infusoria,' ed. iv. 1861, the 

 possession of an undulating oral membrane has been wrongly accredited to this genus. 



Spirostonmm teres, C. & L. 



Body linear-fusiform, from ten to fifteen or twenty times as long as 

 broad, the anterior region slightly narrowest, the anterior extremity rounded 

 obliquely towards the left side only or in an even manner, the posterior 

 border truncate ; peristome-field not reaching to the centre of the body, 

 frequently occupying a third only of its entire length ; endoplast simple, 

 ovate ; contractile vesicle canal-like, extending through the entire length of 

 the body, usually exhibiting a conspicuous posterior dilatation. Length of 

 extended body 1-72" to 1-60". HAB. Pond water among Lemn<z. 



Spirostomum ambiguum, Ehr. PL. XXIX. FIGS. 13 AND 14. 



Body elongate, thread-like, from twelve to sixteen times as long as 

 broad, nearly or entirely cylindrical, the antero-posterior extremities often 

 equally rounded, but the posterior one occasionally truncate ; the peristome 

 extending quite to the centre of the body, frequently beyond this point ; 

 endoplast cord-like, moniliform when fully developed ; contractile vesicle 

 canal-like, extending throughout the whole of the body, often much dilated 

 posteriorly. Length of extended body varying from 1-48" to as much as 

 1-12" or even 1-6". HAB. Pond water among Lemnce. 



