or even single ciliated cells removed from the or- 

 ganism contiime to beat for a long time leads to 

 the conclusion that in the majority of cases the 

 ciliary motion is independent of nervous control 

 of the organism. This is, however, not a general 

 rule since the ciliary motion on small fragments 

 of the hps of the snail, Physa, removed with the 

 attached nerve, soon ceases unless the nerve is 

 stimulated (INIerton, 1923b). Numerous investi- 

 gations give support to the concept that in many 

 invertebrates and vertebrates the nervous system 

 is an effective agent in the control of coordinated 

 activity of cihary tracts (Babak, 191.3; Carter, 

 1927; GothHn, 1920; Lucas, 1935; McDonald, 

 Leisure, and Lenneman, 1927; Seo, 1931). 



Bipolar cells and nervelike fibers immediately 

 below the ciliated epithelium of the gills of fresh- 

 water mussels, Lampsilis and Quadrula, described 

 by Grave and Schmitt (1925), were supposed by 

 these authors to serve as conduction paths for 

 stimuli which they claim regulate and coordinate 

 ciliary movements of the gills of these mollusks. 

 According to their point of view, the ciliated cells 

 of the bivalve gills have a dual control. They 

 may be perfectly autonomous and continue to 

 beat in the complete absence of neural connections; 

 on the other hand, the automatic beat of the cilia 

 may be regulated through supplementary nervous 

 connections in conformity with the state of the 

 organism as a whole. These authors assume that 

 ciliated tissues of fresh-water mussels are botli 

 autonomous and under the control of the nervous 

 system. 



Intracellular fibrillae of the gills of Mya, Lamp- 

 silis, and Quadrula were considered by Grave and 

 Schmitt (1925) to be the conductive paths for 

 coordinating and regulating ciliary movement. A 

 complex system of interconnecting rootlets of the 

 ciliated cells of oyster gills described above 

 (fig. 132) gives additional support to this view. 

 Grave and Schmitt (1925) described also the 

 nervelike apparatus of bipolar cells and fibers. 

 Reinvestigation of the tissues of fresh-water mus- 

 sels by Bhatia (1926) did not support these 

 findings. No such structures were found in my 

 preparations of the gills of C. virginica, or, ac- 

 cording to Lucas (1931), in the gills of Mytilus 

 edulis. Their existence in the gills of fresh-water 

 mussels seems to be doubtful. 



FREQUENCY OF BEAT 



The rate of ciliarj- beat can be observed easily 

 on lateral cilia because of their relativelv large 



size and well-pronounced metachronic wave. Ob- 

 servations must be made on small excised pieces 

 of gill since the position of the lateral cilia on the 

 sides of the filaments makes it impossible to 

 watch their activity on an intact demibranch. 

 In my preparations the filament or a group of 

 them was separated by using fine needles, and 

 kept in a micro-aquarium filled with sea water. 

 The temperature was controlled by circulating 

 cold or warm water in the outside jacket of the 

 microacjuarium. 



The frequency of beat was determined by using 

 a stroboscope of the type manufactured by R.C.A. 

 and sold under the name "Strobotac". The 

 reddish flickering light given off by this instrument 

 is sufficient to observe cilia under a magnification 

 of about 250 X. Readings are made directly 

 on the panel of the instrument by rotating the 

 knobs controlling the frequencies. The instru- 

 ment must be adjusted to the zero point and 

 frequently checked. 



Gradual decline in the frequency of beat on the 

 excised filament becomes apparent after several 

 hours; the disturbance of the metachronism in 

 the preparations kept for more than 24 hours is 

 a sign of pathological conditions. Such prepa- 

 rations should be discarded. 



The frequency of beat varies greatly in different 

 oysters of the same age, origin, and environment. 

 For instance, among the 12 large adult specimens 

 from New England waters tested in August 1956, 

 the range of variation at room temperature of 

 22° to 23° C. was from 16 to 27 beats per second. 

 All the specimens were in excellent condition and 

 appeared normal in every respect. 



In addition, there are sometimes wide variations 

 in the frequencies of ciliary beat in the adjacent 

 filaments of the excised gills. In studies of the 

 effect of temperature and other environmental 

 factors on the rate of beat, therefore, all the read- 

 ings must be made over the same portion of the 

 ciliary tract. This is sometimes diflacult because 

 of the mobility of the excised pieces and copious 

 secretion of mucus which interferes with the 

 observations. 



In the data summarized in table 14 the beat 

 frequencies were recorded in a selected locus of the 

 tract of lateral cilia kept at nearly constant tem- 

 perature. The filaments were taken from the 14 

 different oysters listed in the first column of the 

 table. Observations lasted from 10 to 30 minutes. 

 The maximum range of variation recorded during 



THE GILLS 



137 



