574 ORDER HE TERO TRICHA . 



The order of the Heterotricha includes by far the largest known representatives 

 of the Infusoria, many of its members being not only individually visible to the un- 

 aided eye, but in certain cases, as in the genera Stentor and Spirostomum, attaining a 

 magnitude in their extended state ranging from the one-twentieth to as much as the 

 one-twelfth, or even the one-sixth of an English inch. Although the large com- 

 parative size of the adoral cilia or cirri and presence of conspicuously developed 

 muscular fibrillae in the cortical layer serve to distinguish the members of this order 

 from the more ordinary representatives of the Holotricha, the two groups are separated 

 from each other by but a narrow line of demarcation. In a number of the animal- 

 cules referred to the Holotrichous group even by Stein himself the adoral cilia are of 

 considerably larger calibre than those of the general cuticular surface ; while in many 

 of these again, including notably the Opalinidse, the cortical layer presents a closely 

 parallel fibrillate structure. The Holotrichous genera Panophrys, Cydotricha, Lembus, 

 and Pleuronema have been already referred to as most completely bridging over 

 these two orders. Although including in the Heterotricha all those genera which 

 were originally relegated to it by Stein, it has been considered desirable in this 

 volume to add to them Tintinnus and its allies, hitherto placed by that authority 

 with the Peritricha, but from which they must necessarily be disassociated on 

 account of the ciliation of their cuticular surface. Upon corresponding grounds 

 also the generic types Trichodinopsis, Calceolus, and Codonella demand a similar 

 position among the Heterotricha, each of these, however, differing so essentially 

 among themselves as to necessitate the institution of separate family groups for 

 their reception. Taken as a whole, the various generic groups from Tintinnus 

 onwards in the accompanying tabular view may be said to indicate the line of 

 deviation from the Heterotricha towards the more highly organized order of the 

 Peritricha, from certain of which, excepting for the presence of cilia on the general 

 cuticular surface, they are scarcely to be distinguished. In addition to the affinities 

 of the Heterotricha with the two groups of the Holotricha and Peritricha, just 

 indicated, it will be found that types are not wanting to bring them in close 

 relationship with the remaining order of the Hypotricha also. This affinity is more 

 especially marked in the genus Condylostoma and its allies, in which, as pointed 

 out by Stein, the peristome-neld and adoral fringe of cilia are scarcely distinguish- 

 able from what obtains among the typical Hypotricha, as represented by the genus 

 Oxytricha. 



An analytical key to the families and genera of the Order Heterotricha is here- 

 with subjoined. 



Fam. I. BURSARIAD-ffi, Stein. 



Animalcules free-swimming, persistent in shape, more or less oval, often 

 considerably flattened ; peristome-field excavate, extending obliquely back- 

 wards from the frontal border, widest anteriorly, the oral aperture situated 

 in the posterior and narrowest confines of this region, frequently followed 

 by a well-developed pharyngeal passage ; larger adoral cilia developed in a 

 straight or oblique line on the left-hand margin of the peristome only, 

 not encircling the mouth in a spiral manner ; anal aperture posteriorly 

 situated ; no undulating membrane. 



GENUS I. BURSARIA, Miiller. 



Animalcules free-swimming, broadly ovate, somewhat flattened on one 

 side, anteriorly truncate; peristome-field pocket-shaped, deeply excavate, 

 situated obliquely on the anterior half of the body, having a broad oral fossa 

 in front, and a cleft-like lateral fissure, which extends from the left corner 



