OF THE EOTATOEIA. 



409 



perceive that each one is invested by a delicate sarcolemma. This is well 

 seen at the upper part of the tail, where, on the contraction of the muscle, the 

 non-elastic sarcolemma becomes cornigated, and only recovers its smooth 

 aspect when the muscle becomes relaxed. These rugae of the sarcolemma 

 must not be confoimded with the transverse striae of the muscular fibre." 



Movements op Rotatoeia. — These are very various ; at the same time 

 some varieties are so constant in several genera and species, as to furnish 

 characters of much utility in the systematic distribution of the class. There 

 are two principal modes of locomotion, — one by simple motion onwards, or 

 swimming, with or without rotation of the body on its long axis {e. g. in 

 Brachionus), the other, confined to the family Philodincea, by crawling after 

 the manner of leeches, each extremity of the body being alternately fixed. 

 The latter mode of locomotion is partaken with the first, and the one or the 

 other resorted to at the ^vill of the animal. 



The rotary organ is almost exclusively concerned in producing the uniform 

 swimming movement and in tuimng the animal on itself, whilst the muscular 

 tail acts as a rudder in directing the course. The trochal disk is worked 

 with various degrees of energy and completeness ; when in full action, the 

 velocity attained is very great. 



Usually the Rotatoria swim on the abdomen ; but exceptions occur, as in 

 EospTiora Najas, which, Hke the Phyllopoda, s^vims on its back. Noteus and 

 a few others turn on theii' short axis, or, in common parlance, head over 

 heels. Other exceptional modes of locomotion are met with in Scaridium, in 

 Triartlira, and Pohjarthra (XXXVIII. 30, 32), which have, besides the or- 

 dinaiy swimming movement, the power of leaping or skipping, — in the first, 

 by means of the elongated styliform tail, which can be doubled under the 

 body, and then suddenly relaxed like a spring ; in the two last, by the aid of 

 some rigid bristles, or cin-hi, attached to the body, and acting like the long 

 legs of a flea. A skipping movement is likewise attributed by Ehrenberg to 

 Notommata longiseta, due to its double, long, caudal styles, and an act of 

 rowing, by means of a long lateral spine on each side, to Anurcea biremis. 



The preceding remarks apply to the locomotive Rotatoria ; but the encased 

 species, although unable to change place, have, nevertheless, a considerable 

 power of movement within and about their urceoli. They can extrude the 

 greater part of their body, and bend themselves over the edge of their case, 

 or T^ithdi^aw themselves entii'ely witliin it. Thej^ owe this latitude of motion 

 chiefly to their long pseudopodium or pedicle, which contracts by throwing 

 itself into very numerous and deep wrinkles ; for in none of the attached 

 species is this organ articulated. In comparison with that of the pedicle, 

 the capacity of the trunk of the animal to shorten or contract itself is but 

 small, and its transverse folds few, distant, and collected, mostly towards the 

 posterior extremity. The movements, in fine, of the urceolated Rotatoria are 

 limited to those of extension, retraction, and flexion ; and the extent to which 

 they may be exercised is in direct proportion to the length of the pedicle. Xe- 

 vertheless, when forcibly expelled from its case, which can easily be done 

 without injury to the soft animal, the mature Melicerta swims about with 

 considerable velocity by means of its ciliated rotary disk, — the peduncle being 

 partially drawn up towards the body. 



Although incapable of movement as individuals, a cluster of such as live 

 in compound masses, Conochilus for instance, may float about freely, remind- 

 ing us of the spheres of Volvox. The locomotive Rotifera also enjoy, in a 

 considerable measure, the power of moving their own bodies, — thus frequently 

 altering the relative positions of the various parts, and modifying their general 

 form. Their rotary organ, as already seen, may be extruded or retracted 



