PHYSIOLOGY OF THE INVERTEBRATA. 375 



The Protozoa. 



In these animals a distinct muscular tissue has not been 

 demonstrated, but the sarcode of their bodies is contractile. 

 It may be mentioned, however, that the contractile stalk or 

 peduncle of Vorticella contains a differentiated, longitudinal 

 muscular fibre, which is capable of contracting so as to give 

 the stalk the form of a spiral. 



The organs of locomotion in the Protozoa are the pseudo- 

 podia, flagella, and cilia. 



In the Amceba it is by means of pseudopodia that the 

 animal moves ; "it emits them in the direction in which it is 

 going, then it retracts them, while other parts of the mass 

 are in their turn elongated. The whole body moves by 

 creeping. This organism in moving has the aspect of a 

 drop of oil moving along. To explain the mechanism of this 

 movement, it must be supposed that the extended pseudo- 

 podium seizes some point of support with its free end, then, 

 in contracting, draws the entire mass of the body up to this." 



According to M. Rouget the retraction of the pseudopodia 

 is the analogue of muscular rigidity, the emission of these 

 organs being due to internal pressure ; and that the hyaline 

 substance of the pseudopodia is a kind of hernia of the 

 ectosarc, " resulting from a diminution of the elastic resistance 

 at the point where each pseudopodium appears, with an 

 increase of elasticity in those parts of the ectosarc where 

 pseudopodia are not produced. When the elastic tension of 

 these parts diminishes, and returns to its original state the 

 pseudopodium re-enters into the mass." M. Rouget further 

 states that in Amceba terricola, the most external portion of 

 the ectosarc shows " striae of a granular appearance which 

 may be identical with the striae or contractile fibrils of the 

 ectosarc of the ciliated Infusoria — Stentor, Spirostomum, 

 Parammcium, &c." 



The Gregarina moves in a worm-like, gliding, fashion, but 



