MICROSCOPIC OBJECTS. 53 



Fig. 105. Polycystina : Fossil Shells from Barbadoes, x 100. 



These beautiful shells are closely related in their classifi- 

 cation to the Foraminifera, being grouped with them under 

 the head of Rhizopoda, which may be described as consisting 

 of organic structureless bodies, as in Amoeba. But in the 

 Foraminifera, Polycystina, &c. these bodies are confined in 

 a certain degree within their shell, although they send out, 

 through the small holes in the shells, retractile processes, 

 which answer in their functions to the legs of insects &c., 

 being organs of locomotion. The species are mostly fossil, 

 and are chiefly found in the West-Indian Islands &c. The 

 remarkable beauties of these forms might well be taken 

 advantage of, as patterns for jewellery and other manufac- 

 tured articles. They, like the Foraminifera, may be mounted 

 in balsam, or in a dry opaque cell &c. However they be 

 mounted, they will well repay the trouble bestowed on them. 



Fig. 106. Section of Sponge (Spongia usitatissima), x 120. 



On examining this drawing of the horny fibres of the 

 Common Sponge, the reason of their retaining so much water 

 will be evident. The fibres will be noticed to be of a very 

 elastic structure, which, by compression, discharge the water, 

 and on their reflex action absorb it. By the capillary action 

 of the numerous fibres they also retain the water until the 

 sponge is pressed. Sponges, in their living state, are covered 

 with an amorphous jelly-like substance similar to that of the 

 Amoeba (fig. 101) j and if this substance be examined under 

 the microscope, its motions will be found nearly identical. 

 Sponges are mostly marine animals; but there are a few 

 freshwater species to be found in England. 



No doubt sponges live by absorbing their prey, in the same 

 manner as the Amoeba. Sections are best mounted in fluid. 



