Both swimming and attachment behaviour of immature female Brachionus calyciflorus (Pallas) from a young orthoclone were analyzed under four experimental conditions : with and without algae (Chlorella), and with fed and starved individuals . Eighteen hours before the recording, two random groups were formed . To the first group, Chlorella was added . The second was placed in pure water . Both were filmed in red light to avoid a phototactic effect . Video-recorded swimming behaviour of B . calycii locus was analyzed by automatic tracking . Each B . calyc focus was filmed alternately in the presence or absence of Chlorella for five periods of 15 min . Under indentical light and temperature conditions the number of animals attached to the culture dish was measured with both starved and fed females . We observed a significant difference in swimming paths depending on the presence or absence of food, and the physiological state of the individuals . In the presence of Chlorella the linear speed was lower and the angular speed was greater . This increase in angular speed was more pronounced when females were subjected to a period of starvation before the experimentation . The attachment behaviour increased in the algal environment . This modification of swimming behaviour enables females to increase the time spent in a favorable food environment . 95

The skeletal muscles of rotifers are monocellular or occasionally bicellular. They display great diversity of cytological features correlated to their functional differentiation. The cross-striated fibers of some retractors are fast contracting and relaxing, with A-band lengths of 0.7 ptm to 1.6 [tm, abundant sarcoplasmic reticulum and dyads. Other retractors and the circular muscles are tonic fibers (A band > 3 gm), stronger (large volume of myoplasm) or with greater endurance (superior volume of mitochondria/ myoplasm). All of these retractor muscles are coupled by gap junctions and are innervated at two symmetrical points; they constitute two motor units implicated in withdrawal behaviour.The muscles inserted on the ciliary roots of the cingulum control swimming. They are multi-innervated and each of them constitute one motor unit. They have characteristics of very fast fibers; the shortest A-band length is 0.5 pm in Asplanchna.All the skeletal muscles of bdelloids are smooth or obliquely striated as are some skeletal muscles of monogononts. These muscles are well suited for maximum shortening and are either phasic or tonic fibers.All rotifer skeletal muscles originate from ectoderm and contain thin and thick myofilaments whose diameters are identical to those of actin and myosin filaments in vertebrate fast muscles or in insect flight muscles. There are no paramyosinic features in the thick myofilaments. The insertion, innervation, coupling by gap-junctions and other cytological differentiations of rotifer skeletal muscles are reviewed and their phylogeny discussed.

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This work explores the effect of darkness on the swimming behaviour of females of the rotifer Asplanchna brightwelli . Females were filmed in flat dishes alternately in white light (WL, 5000 µW cm -2) and in infra-red light (IR, 155 µW cm -2 ), each female for a total of eight successive periods of fifteen minutes per period . An automated tracking system was used to describe the swimming path of each female : twenty five x and y coordinates of the center of gravity of the animal per second, in a discrete space of 512 x 512 pixels . Indices characterizing the swimming performances of the females were then calculated : linear speed, angular speed and other angular parameters of the tracks . A Principal Component Analysis of swimming characteristics discriminated between WL tracks and IR tracks . Females swam slower and turned more in darkness than in light . These results show that beside a positive phototactic response, there is a photokinesis which increases the dispersion of animals in the light . 101

A study of the anterior sensory receptors of male and female Asplanchna brightwelli by scanning electron microscopy reveals some important differences in the region surrounding the mouth . In the male, the ventrolateral sensory bristles, the pseudotrochus, the inner and the outer buccal tufts and the mastax receptors are absent . The oral receptors are reduced .Transmission electron microscopy of these receptors shows that they consist of ciliated sensory cells surrounded by epithelial supporting cells . The distal ends of the cilia of the mastax receptors are modified ; the cilia of the other receptors differ only in their length and rootlet structure from the locomotor cilia of the cingulum .A consideration of the feeding behaviour of Asplanchna leads us to suppose that these sensory cilia function in mechanoreception and in chemoreception .