38 p. KIDD. 



feebly, and possesses clearly a very great flexibility and con- 

 siderable elasticity, as may be easily proved in the tentacles 

 of the Hydra. Consequently it is probably very rich in 

 water ; and, as a rule, the amount of water contained in 

 organized bodies and their double refracting capacity vary 

 inversely. 



Since the double-refracting capacity of the anisotropous 

 layer of the Hydra between the ectoderm and endoderm is 

 very considerable, and may be estimated as not less than 

 that of the double-refracting substance of striped muscle, it 

 is probable that apart from the supporting lamella the muscle- 

 fibrils of the Hydra are themselves double refracting. And 

 this can be directly proved after maceration for a short time 

 in perosmic acid one quarter per cent., for then the muscle- 

 fibrils can be separated from the supporting lamella by 

 means of needles, and still show traces of double refraction. 

 Similar results were obtained in the case of the contractile 

 layer between the ectoderm and endoderm of a Hydractinia. 

 Distinct fibrils could not be distinguished in this layer. 



2. Contractile Fibres and Protoplasm of Infusoria, Zoo- 

 thamnium arhuscula. — The stalk muscles of a living un- 

 injured specimen showed a definite fibrillation, visible with 

 a magnifying power of 150, and refracted light tolerably 

 strongly. 



During contraction the fibrils became visibly shorter and 

 thicker. Engelmann considers that the stalk muscles of the 

 Zoothamnium are not semi-fluid during life, but consist of 

 tolerably definite contractile fibrils. The bands of muscle 

 were contained in a structureless, double -contoured, and 

 highly refractive membrane. On one side of the muscle the 

 membrane fitted closely, but on the other loosely, so as to 

 give rise to a space between it and the muscle. This space 

 was filled with a colourless substance Avhich, at the end of 

 the branches was quite clear, but at a greater distance from 

 the animals became increasingly richer in very small, round, 

 tolerably refractive granules, which at last assumed the 

 character of rather coarsely granular protoplasm. By the 

 movements of the muscle these granules Avere shifted about 

 as if in a semi-fluid mass. No active contractility was 

 observed in this tissue. 



With polarised light the stalk muscles showed themselves 

 to be double-refracting, the degree of double refraction being 

 equal to that shown by the muscle of vertebrates and insects 

 Avhen mounted in Canada balsam, and distinctly greater 

 than that of the leg muscles of Musca examined in the 

 living condition in salt solution or serum. The fibrils of the 



