56 PROCURING OBJECTS. 



even small fragments of ivory, the animal to which the tooth belonged* 

 Shells also present beautiful appearances beneath the microscope. 

 The shell of the echinus, or sea urchin, for example, is found to be 

 composed of a network of calcareous matter, sometimes forming a 

 series of plates parallel to each other, and connected with little pillars 

 proceeding from one surface to another. In the spine, with which the 

 animal is covered, this network has a most beautiful appearance. The 

 shells of moluscs have been shown by the microscope not to consist of 

 mere masses of calcareous matter, as a piece of limestone is, but are 

 distinguished each by some peculiarity of structure, which the micro- 

 scope exhibits to us. Primarily the shell of a molluscous animal is 

 composed of cells of animal matter, in which are contained calcareous 

 matter. In many cases, the shells are of a prismatic form, and the 

 internal matter takes its shape from the cells. Here again the 

 naturalist, by seeing the smallest fragment of shell, or even a little of 

 the calcareous dust left when the membrane was discharged from it, 

 can tell to what tribe of molluscs it has belonged. 



59. CIRCULATION IN ANIMALS AND PLANTS. One of the most 

 interesting active phenomena exhibited by the microscope, is the 

 circulation of the nutritious fluids in animals and plants. In the 

 former, the globules of blood may be seen passing rapidly along the 

 capillary ends of arteries into those of the veins when the intervening 

 membrane is sufficiently diaphonous, as in the ear of the young mouse, 

 the fins and tail of the carp, gold-fish, stickleback, tadpole, and of 

 most small fish, and in the web between the toes of the frog, lizard, 

 etc. etc. It need hardly be remarked, that in order to observe this 

 phenomenon, the animals should be alive and fastened securely upon 

 the stage of the microscope the part to be examined, stretched before 

 the object-glass, and a strong light directed through it. 



Mr. Pritchard states, that in the Arachnoida, (spiders,) the circu- 

 lation may be observed very distinctly, the currents of dark globules 

 passing rapidly at each pulsation of the dorsal vessel. In the antennae 

 and wings of terrestrial insects, it has also been seen when they have 

 just emerged from the chrysalis, as in the Perla Viridis, and Semblis 

 Bilineata. In several aquatic larvae and small curstacea, the circulating 

 fluid appears to traverse the limbs, antennae, and tails, and thence 



