OF THE KOTATORTA. 399 



has very many arguments in its favour, and in support of which I may 

 adduce the structure of the rotaiy organ of Pterodina. This species, belong- 

 ing to the family of BracMoncea, has its free projecting lobes furnished with 

 a double row of cilia, analogous to what occurs in Megalotrocha. That the 

 wheel organ of Philodincea also is referrible to the same type, is evident 

 from the account Huxley gives of it." 



Cohn {Zeitschr. 1855, vii. p. 437) describes two complete rows of cilia, 

 besides five or six special ciliary bundles, on the head of Hydatina senta 

 (XL. 1). On the outer margin is an unbroken row of long and fine cilia, 

 extending thence into the oral fissiu"e, and still further, into the oesophagus. 

 Within this circle is an interrupted one formed by 6 or 7 (Ehrenberg counted 

 11) bundles, having few or many broader and longer cilia, nearly resembling 

 the setae on Stylonychia, and supported on as many cushion-like eminences. 

 Lastly, the thii^d series is unbroken like the first, and composed of finer cilia, 

 disposed in a quincimcial manner in two lines. All the parts of this ciliary 

 apparatus work harmoniously together in eftecting the movements of the 

 animal or in securing the capture of food. 



The figure of the trochal disk (XXXYIII. 14, 15, 20, 21) varies exceed- 

 ingly, as the quotation from Leydig indicates, and is especially influenced by 

 the addition of supplementary ciliated eminences. In Megalotrocha (XXXII. 

 374-378) the disk is horse-shoe-shaped ; in Melicerta it is petaloid, or, as 

 Prof. Williamson called it, flahelliform ; in Rotifer it is seen under two forms, 

 according to its degree of expansion, either as a single conical eminence, or, 

 when completely unfolded, as two cylinder-like processes, one on each side 

 of the head, apparently whorling like two wheels. In the family Brachio- 

 ncea (Ehr.) accessory disks or processes give rise to much complication 

 (XXXIX. 15-22). Ehrenberg described this family as having two ciliated 

 organs — a central one of three parts, and a lateral one of two, — the latter 

 being the time wheel organ, and the former, frontal processes which are stiffly 

 extended whilst the rotary organ is in action. An appendage such as that 

 last named, in Koteus, he designates a three-lobed ciliated brow. 



Exceptional or aberrant forms of the cihated disk are seen in Floscidaria, 

 in Steplumoceros, and in Lindia (XXXIX. 1, 3). In the first, the head 

 (XXXYII. 1) is surmounted by five ciliated flattened lobes, ending in knob- 

 like processes which bear very long, divergent, non-vibratile haii^s or cilia 

 of uniform thickness {see woodcut, Part II.). " These exceptional cilia," says 

 Dr. Dobie, '' are slowly moved and spread out by the contractile substance 

 of the lobes of the rotary organ." In Steplianoceros, the departure from the 

 normal structure is still greater (XXXYII. 1), — so much so, that the 

 ciliated appendages have no claim to the title of a rotary organ. Five long 

 arms extend from the head, Kke five tentacles, covered by cilia in rings (ver- 

 ticellate ciHa). These arms not only act like a common trochal disk by pro- 

 ducing a vortex directing aU particles within its range to the mouth, but also 

 as organs of prehension, closing themselves on any larger object which may 

 come within their grasp. This ciliated armature around the head bears a 

 close resemblance to that of the cilio-brachiate Polypes or Bryozoa, to which 

 class of animals, indeed, several distinguished naturalists have referred the 

 genus Stephanoceros, not merely on account of this one affinity named, but 

 also from several other coincident characters. A third peculiar form of rotary 

 organ has been recently pointed out by Cohn in Lindia {Zeitschr. 1858, p. 

 284). It takes the form of a club-shaped process on either side of the head 

 (XXXIX. 1, 3), having its extremity somewhat expanded and spherical. 

 Cilia exist only on the round summits of these processes ; there is no whorl 

 around the margin of the head, none elsewhere on the body ; and this ex- 



