THE GENERAL PHYSIOLOGY OF THE PROTOZOA 199 



spicules and various skeletal structures ; the shells, houses, etc. 

 adhesive substances or stalks in sedentary forms, as, for example, 

 the non-contractile stalks of many Vorticellids (p. 441) ; and the 

 cysts or envelopes secreted round the body, such as the sporocysts, 

 etc. The pseudopodia of many Amoebaea, such as Difflugia, are 

 covered by a sticky slime which enables the animal to adhere to 

 surfaces over which it creeps, and which can be drawn out by 

 contact with a glass rod into threads, like the mucus of a snail 

 (Rhumbler, 34). In Foraminifera and Heliozoa the pseudopodia 

 appear to secrete a substance which holds the prey fast, and at the 

 same time kills it, as already mentioned. Some Protozoa for 

 example, gregarines leave a trail of mucilaginous substance behind 

 them as they move forwards, and by some authors this sesretion 

 has been regarded as the mechanism by which locomotion is effected 

 (p. 327). Internal secretions in connection with the digestive 

 function have been mentioned in a previous section. Arcella has 

 the power of secreting gas-bubbles in its protoplasm for hydrostatic 

 purposes (compare also the Radiolaria, p. 252). 



4. Transformation of Energy (a) Movement. The different motile 

 organs of Protozoa have been described above. Considered from a 

 morphological standpoint, the protoplasmic body may exhibit, in 

 the first place, no specially differentiated organs of movement, 

 which then takes the form of currents and displacements in the fluid 

 protoplasm itself, manifested externally in the form of pseudopodial 

 processes or flowing movements of the entire body, internally as 

 streaming movements in the protoplasm. Secondly, there may be 

 special organs of movement, either external, in the form of vibratile 

 organs, such as cilia, flagella, or undulating membranes ; or internal, 

 in the form of contractile fibrils or myonemes. 



Different as pseudopodia may appear at first sight from vibratile 

 organs, such as cilia or flagella, there is nevertheless a very gradual 

 transition from the one type to the other (see p. 53, supra). Of 

 pseudopodia there are two chief types of structure the lobopodia, 

 in which a fluid core of endoplasm is enveloped by a superficial layer 

 of stiffer ectoplasm ; and the axopodia, in which, on the contrary, a 

 secreted axis of rigid or elastic nature is covered by a more fluid layer 

 of protoplasm. The axopodia are connected by transitions both 

 of structure and movement with organs of the vibratile type. In 

 both flagella and cilia the structure consists of a firmer elastic axis 

 covered over by a more fluid superficial layer (pp. 52, 54) ; many 

 axopodia exhibit swinging, nutating, or bending movements differ- 

 ing only in degree from those of flagella (p. 51). There are grounds 

 for believing the one type of organ to have been derived phylo- 

 genetically from the other. 



The streaming movements of protoplasm have been the subject 



