OF THE ROTATOEIA, 401 



muscles arising within the abdomen, which draw do^Tiwards, and therefore 

 inwards, the disk to which they are attached. At the commencement of 

 their traction, they draw together the sides of the ciliary Avhorl, then pull 

 inwards the cilia, which are previously collected in a cylinchical manner, and 

 at last cause the inversion of the integument immediately beneath the disk, 

 when the now anterior extremity of the body contracts itself upon the in- 

 cluded parts. This process of involution may be arrested by the animal at 

 any stage ; thus, sometimes it is stayed when the cilia are grouped together 

 in a cylinder-like heap, and still project from the head like a pencil ; or, 

 as above mentioned, the cilia may be withdi-awn, and some process or an- 

 tenna be left protruding. The collection of the cilia into a brush-hke group 

 during the process of retraction is well exemplified in the long cilia of Flos- 

 cularia ; and in Rotifer and PliUodina we have a special example of the pro- 

 trusion of a ciliated process during the involution of the major part of the 

 trochal disk (XXXYIII. 1). In the genera last cited, this median process 

 serves as the anterior organ of progression when the animals advance in a 

 leech- like manner, and disappears when the paii' of trochal organs are 

 evolved and the crawling movement is changed to swimming. 



The retraction of the trochal disk we may suppose to be controlled by the 

 will of the animal to arrest its motion or to avoid danger. Another motive 

 is conceivable, especially in the case of the attached species : for the ciha, 

 when in active operation, attract every sort of particle within their vortex — 

 as well those appropriate to nutrition as others noxious or which have 

 been lately discharged and still float about the animal ; hence it may be ne- 

 cessary to arrest their action, withdraw the disk, and close all access to the 

 interior, until these unfit substances are floated away and have been replaced 

 by others. 



The ciliated mechanism of the head is, as just hinted, the active agent in 

 procuring food, by di^agging within its vortex the nutritive particles in reach, 

 and transferring them to the mouth, which is so situated that the current 

 produced sets directly into it. "Where the ciliaiy wreath is double, as in 

 Melicerta, ''the food" (to use Prof. Williamson's description) " that reaches 

 the mouth is whirled around the wheel- organs along the groove that sepa- 

 rates the two circlets of cilia ; and since these circlets diverge near the 

 ' chin ' (or fifth ciliary lobe), the mouth being located between them, the 

 food is necessarily conveyed directly to the latter organ. The two sets of 

 marginal cilia, by bending towards each other whilst in motion, almost con- 

 vert this groove into a sinus, especially in the two large segments." But 

 besides locomotion and nutrition, the rotary apparatus must be admitted to 

 subserve the function of respiration, both by its own delicate stnicture, and 

 by its action in constantly renewing the water around the animal ; also, by 

 forcing fluid within the alimentary canal, it may serve to aerate and renew, 

 by endosmosis, the fluid in the general cavity surrounding the viscera. 



In the fixed species of Rotatoria the rotaiy organ can have no locomotive 

 use, merely subserving the functions of nutrition and respiration. In addi- 

 tion to the rotary organ, the head is often beset with various appendages 

 in the shape of stjiiform and tubular processes, lobes, disks, uncini, and 

 spines. These are situated either witliin the cii'cle of the ciliary wreath on 

 its margin, or immediately external to it. Examples of tapering, styliform, 

 and bristle-hke processes are found in Notommata Myrmeleo, Monocerca bi~ 

 cornis, in Si/nchcefa, Monostyla, Brachionus, and others. On the head of Cono- 

 cMliis are four stout wart-like elevations. In Polyarthra platyptera two long 

 bristles project from near the mouth, each bent on itself midway at a right 

 angle (geniculate). Dujardin describes, in his genus CoJurella, an uncinate 



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