i 4 8 COMPARATIVE ANATOMY 



distance from the centre of the body, in the endosarc. These 

 needle-like supports of the pseudopodia form the skeleton of 

 the animal. They end peripherally in extremely fine points ; 

 proximally they are somewhat thicker, and their internal ends 

 are wedge-shaped. They give the stiff ray-like appearance to 

 the pseudopods ; but they are not stiff, but elastic and easily 

 bent, being formed of an orga-nic substance which has been 

 called elastin. This substance may be absorbed by the 

 protoplasm and the whole pseudopod withdrawn, and if the 

 pseudopod is again protruded its axial rod is reformed. The 

 radiating rods are clothed with a layer of protoplasm, thick- 

 est at the base of the pseudopod, and tapering to a very 

 fine thread at its end. The protoplasm does not invest the 

 skeletal rod as an even coat, but here and there it may show 

 swellings and prominences which come and go, and it may 

 easily be seen, on close examination, that the protoplasm of the 

 pseudopods exhibits the streaming movements which have 

 already been described as characteristic of the protoplasm of 

 many cells. Granules may be seen floating up one side of a 

 pseudopod and down the other ; and a similar streaming of 

 granules can be seen in the protoplasmic walls of the vacuoles, 

 both of ectosarc and endosarc. If some small and active 

 infusoria are introduced into the drop of water in which 

 an Actinosphcerium is imprisoned under observation,, its mode 

 of catching and ingesting its prey may be seen. When the 

 infusorian swims among the pseudopods and touches them 

 they bend suddenly inwards so as to enclose it in a trap ; the 

 infusorian struggles for a while, but appears soon to be 

 paralysed, though it is not clear why it is. As its struggles 

 become less violent, knots of protoplasm appear on the distal 

 extremities of the pseudopodia enclosing it, and these travel 

 centripetally down the axial filaments, drawing the infusorian 

 with them till it reaches the ectosarc into which it is passed, 

 and so through the ectosarc into the endosarc where it is 

 surrounded by a food-vacuole. Food may be ingested at 

 any part of the body, and it is evident that the stiff radiating 

 pseudopodia form a trap-like apparatus, enabling the inert- 

 looking Actinosphnerium to capture organisms much more 

 agile than itself. The active bending movements of the 

 pseudopods are manifestations of the contractility of the 

 protoplasm, and contractility is further exhibited by the 



