BY WILLIAM J. BYRAM. 19 



precision of the shapes. [Arachnoidiscus.] That is the beautiful 

 arachnoidiscus, or spider-web disc, the reason for the name being 

 obvious, [Pleurosigma.] That photomicrograph is the beautiful 

 pleurosigma under a high power. And I should like to direct 

 your attention to the small portion at the side turned back so as 

 to show the two sets of markings. It is a pretty feat of mounting 

 that, when we consider that the whole object is a minute point 

 invisible to the naked eye. The diatomaceae occur in every part 

 of the world in countless myriads, and how numerous these 

 minute organisms have been in the geological past you will 

 realise when you learn that they occur fossil to such an extent 

 that whole strata consist of little else, and whole mountains are 

 composed of them. The slide now projected shows some of this 

 diatomaceous earth. 



The enclosure of the protoplasm in a test or shell occurs in 

 a variety of other forms. In the animalcule called Gromia, the 

 shell or carapace is of chitin, a peculiar horny nitrogenous 

 substance, of which the wing cases of certain insects are also 

 composed. In the shell there is only a single small orifice at one 

 end, and through this the protoplasm streams forth abundantly, 

 completely investing the shell externally, and branching and 

 re-branching and interlacing so as to form a delicately 

 complicated network. The carapace is the home-centre, and in 

 states of quiescence the whole of the protoplasm is withdrawn 

 into it. The creature is like an amoeba that has acquired a shell. 



Another example of the enclosure of the protoplasm in a test 

 or shell is seen in the beautiful little creature, Clathrulina 

 elegans. That name sounds formidable, but the word Clath- 

 rulina simply means little trellis or grating, and you will at once 

 see that the name has been given to it on account of the 

 perforations in its shell. The shell is placed upon a stalk, which, 

 like the shell itself, is composed of silex or flint. The creature 

 is a speck of protoplasm, and through the apertures of the shell 

 it puts forth rays like the sun-animalcule. This comparison with 

 the sun -animalcule is no fancied resemblance, as the method of 

 reproduction shows. At times numerous small oval masses of 

 protoplasm are formed within the shell. They escape, acquire 

 flagella and swim about actively. Then they assume the form 

 of free sun-animalculae, and ultimately gradually acquire the 

 silicious shell and stalk. From these forms the transition is 



