BRYOZOA. 97 



protrusion is completed, the crown of tentacles expands and their 

 cilia commence vibrating. 



The advantage to Physiology of the researches of the comparative 

 anatomist in the minute forms of animal life, is often very great, in 

 consequence of the favourable conditions which the transparency of 

 the integument, and the distinctness of the contained parts of such 

 animalcules, afford for the direct observation of some of the most 

 recondite and important vital actions. As regards the Bryozoa, the 

 muscles are, as it were, naturally dissected or separated into their 

 component filaments. Each filament generally presents a small knot 

 upon its middle part : this is most apparent when the filament con- 

 tracts, at which time the whole filament is obviously thicker. When 

 the action ceases and the filament is relaxed, the distance between its 

 fixed points being diminished, as happens to the longitudinal fibres 

 when the polype is retracted into its cell, such fibre falls into undula- 

 tions. The thickening of the muscular fibre in the act of contraction, 

 and its folded state when it relaxes, before the antagonising muscles 

 have restored the extremities of the contracted fibre to their ordinary 

 distance, has been observed in other low organised animals, as small 

 Filarice. The higher organised subjects selected by MM. Prevost 

 and Dumas, were less favourable for this delicate experiment, and 

 they consequently mistook the zig-zag relaxation of the muscular fibre 

 for its act of contraction. 



No trace of a nervous system has yet been detected in the Bryozoa; 

 but the reaction of stimuli upon the contractile fibre is a striking 

 phenomenon. The animal retires into its cell on the slightest alarm, 

 and refuses to expose itself to water which has become in the least 

 degree deteriorated. Dr. Farre has observed the creeping of a very 

 small animalcule over the top of one of the closed cells to be fol- 

 lowed instantly by the shrinking of the soft parts beneath. But the 

 nervous system is indicated in these little polypes by something more 

 than reflex phenomena : they seem to exercise a certain caution before 

 emerging from their cells. One or more of the tentacles have been 

 seen to be protruded and turned over the side of the cell, as if to 

 ascertain the presence or absence of an enemy. 



I must now proceed to describe these tentacula (c, c), which are the 

 means by which the Bryozoa obtain their food. They differ con- 

 siderably from the corresponding tentacula in the Hydrozoa and An- 

 thozoa, in being stiff'er and provided with vibratile cilia. These cilia 

 are arranged on opposite sides of the tentacle, along which sides they 

 occasion, by their active vibration, opposite currents of the surrounding 

 water. In some species a few fine hair-like processes, which are mo- 

 tionless, project from the back of the tentacula. The action of the 



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