398 GENERAL HISTOET OF THE INFIJSOEIA. 



the first of these groups he gave the name of Sorotivcha, to the second Poly- 

 troclia, and to the last Zygotrocha. The further subdi\dsions which he formed, 

 and the names he applied to the varieties of the rotary organ, will be ex- 

 plained in the section on Classification. The belief in actually compound 

 trochal disks has been shared by nearly all observers, and both Perty and 

 Siebold adopt it along with Ehrenberg's classification. On the other hand, 

 it is denied by Leydig, who affirms that the disk is never divided into 

 such secondary wreaths or lobes, but always constitutes one continuous 

 margin, variously extended and folded, and, it may be, furnished with inde- 

 pendent accessory ciliated disks. This able writer remarks — " It is only to 

 the exceptional genera StepJianoceros and Floscularia, that Ehrenberg's term 

 Polytrocha can be rightly applied. In truth, an observation recorded by the 

 great micrographer himself negatives his hypothesis of polytrochous division, 

 that, viz., where he applied strj^chnia to the rotary organ of Hydatina, which 

 became thereby reduced to a simple whorl of cilia." 



The various degrees of complication assumed by the trochal disk are thus 

 detailed by Leydig : — " It forms a simple ciliated margin around the mouth 

 of Notommata tardigrada ; in Stephanops (XL. 8, 10) it is wider, more pro- 

 minent, and triangular ; in Euchlanidota, Polyarthra (XXXVIII. 30), Di- 

 glena, Distemma, Hydatina (XL. 1), Pleurotrocha, and others, it occupies the 

 entire periphery of the head, and is not at all, or but very slightly, elevated 

 as a distinct disk above it ; in Notommata Copeus, N. aurita (XXXVI. 4), 

 and in SyncJiceta, it is enlarged and elevated as a distinct disk on each side of 

 the head, forming the " ears " so called by Ehrenberg ; in other instances it 

 is enlarged, and projects on the ventral sui'face of the animal like a ciliated 

 trunk or proboscis. A higher development is seen in Brachionns (XXXIX. 

 15-18) and Philodina, where the ciliated border is involuted and extended 

 upwards laterally (XXXVIII. 2) ; and lastly, in Megalotrocha, Lacinularia, 

 Melicerta (XXXVI. 1 ; XXXVII. 17), and Limnias (XXXVI. 2), the high- 

 est complexity is reached, and the trochal disk appears to be an appendage 

 surmounting the head, expanded in the form of a sinuous or lobed ciliated 

 margin." In the variety last mentioned, Mr. Gosse speaks of the expanded 

 lobes under the name of " petals." 



The row of cilia flinging the rotary organ is often single, but in several 

 species is double, and even treble. Mr. Huxley has noticed its double con- 

 dition in Lacimdaria sociaJis. To quote his description — " The edge of the 

 disk has a considerable thickness, and presents two always distinct margins, 

 an upper and a lower, of which the former is the thicker, and extends beyond 

 the latter. The large cilia are entirely confined to the upper margin, and 

 form a continuous horse-shoe-shaped band, which, upon the oral side, passes 

 entirely above the mouth. The lower margin is smaller and less defined than 

 the upper ; its cilia are fine and small, not more than one -fourth the size of 

 those of the upper margin. On the oral side this lower band of ciha fonns 

 a V-shaped loop, which constitutes the lower and lateral margins of the oral 

 aperture. About the middle of this margin, on each side, there is a small 

 prominence, from which a lateral ciliated arch runs upwards into the buccal 

 cavity, and, below, becomes lost in the ciha of the pharynx. The aperture 

 of the mouth, therefore, lies between the upper and lower ciliated bands 

 (XXXVIII. 21)." 



Prof. Williamson has signahzed a like arrangement in Melicerta (XXXVII. 

 17), and LeycHig in. Br achionus, Pterodina (XXXVIII. 29), and Megalotroch<xa. 

 The latter writes — " On the free surface of the head oi Brachionus (XXXVIII. 

 14, 15), two lateral and and a median lobe elevate themselves, which Huxley 

 compares to the two ciliated borders of Lacinularia, — an interpretation that 



