OF THE EOTATOEIA. 407 



process much depends on the influx of fluid forced into it by a general trans- 

 verse contraction of the body which is seen to precede it. The extension of 

 the body, after having been shortened by the contraction of its longitudinal 

 muscles, is chiefly due to the elasticity of its integument, which has an in- 

 herent tendency to constrict itself or to lessen its diameter. Prof. Williamson 

 dissents from this explanation, believing the extension to be due to the cir- 

 cular muscular bands, as in the pseudopodia of the Echimis and starfish, or in 

 the tnmk of an earthworm. The shortening of the body is provided for by 

 the sliding stnictui^e of its segments, and by the wrinkling (XL. 1) of its sur- 

 face (XXXIX. 1-3), sometimes by both these modes together, at others by 

 one alone. Even where its length is diminished by the formation of mere 

 folds of the skin, those folds are constant in position and arrangement. 

 Longitudinal folds pretty regularly disposed occur in the softer-skinned 

 varieties — for instance, in various species of Notommata and Hydatina. 



Muscles supphdng special organs are seen in connection with the trochal 

 disk, the maxillary head and jaws, the alimentary canal, and the reproduc- 

 tive apparatus (XXXIX. 7). Excepting the muscles moving the rotary 

 organ, these will be best described in the account of the organs with which 

 they are connected. 



The trochal disk, and, indeed, the whole head supporting it, is constricted, 

 corrugated, contracted, and moved from side to side by considerable muscles, 

 extending from it to the maxillae, and to the sides and posterior boundary of 

 the abdominal cavity (XXXYIII. 28 a) ; special muscular threads act upon 

 particular lobes, prominences, or processes which may extend from the head 

 or its ciliated disk. In the trochal disk of Melicerta, Prof. Williamson de- 

 tected interlacing threads which he supposed to be muscular ; and Mr. Gosse 

 has remarked in the same animal '' a series of five or six annular threads 

 set in the inner skin, which are probably muscular, and aid in the complex 

 movements of the head." Some of the interlacing threads, which Ehrenberg 

 described in several Rotatoria (as, for instance, in Lacilunarici), and which 

 at one time he regarded as vascular, at another as a nervous or muscular 

 network, probably were muscular, although most of them were merely fibres 

 of connective tissue. 



The extrusion of the head and trochal disk, after retraction, is principally 

 effected, as in the case of the pseudopodium, by the elasticity of the integu- 

 ment, consentaneous with the relaxation of the muscular contraction, — this 

 elasticity serving to unroll the involuted head and trochal disk, and to expand 

 their parts, and, by its general operation on the body, to elongate the whole 

 figure, and thereby press the contained fluid forward and backward against 

 the retracted organs, so as to push them out. Prof. Williamson would also 

 attribute the protrusion of the head to the action of the circular muscles, 

 as he does not think there is sufficient proof of such elasticity independently 

 of muscular fibres. The retracted head and appendages of the Bryozoa are 

 thrust outward in a similar manner. 



The cilia of the trochal disk have generally been assumed to be seated on 

 a muscular mass, forming the cushion-like contractile thickenings on the 

 head of the Rotatoria (XXXVI. 93). These structures display, according to 

 Dujardin, no distinct muscular fibres ; but in the opinion of others, such are 

 present. Ehrenberg, as before stated, went so far as to imagine, not merely 

 a network of muscular fibrils moving the entii'e apparatus, but also a series 

 of four muscles at the base of each cilium moving it in every direction. Such 

 an array of definite muscles to move an almost imperceptible organ, is not 

 only entirely hypothetical, but most improbable. Leydig, on the other hand, 

 opposes the idea of the muscular natiu-e of the trochal disk, and regards 



