468 Dr. Wallich on the RJiizopods, 



which I wish to draw attention in connexion with the forma 

 to wliich reference has been made in the present and last 

 month's instalment of this paper. The first figure in my list 

 represents in outline one of the simplest and most common 

 forms of Difilugian test, namely the mitriform — simplest 

 because it approaches most closely to the spherical form (which 

 there is reason to regard as the archetype of the entire 

 Difilugian series) and constitutes also the earliest stage of 

 most of the varieties of Difflugian tests which in the adult stage 

 bear scarcely any resemblance to it : the tendency to variation 

 in outline being, according to my view, neither resident in the 

 animal nor in the test, hut in the varying external conditions^ 

 which give the first impetus to change of figure when the 

 young test has not as yet lost its purely membranous 

 character, and is therefore still perfectly plastic. Once set 

 in motion, the same external forces or influences (for some 

 are really forces, whereas others are merely influences) 

 continuing to act in the same direction naturally tend to 

 stereotype and extend the characters first impressed on the 

 young test. It is in this way, I contend, that whereas we 

 have but one type of animal to deal with, we have an 

 almost infinite variety of tests. It follows, therefore, that if 

 we attempt to do more than group together the various 

 most closely related varietal forms and kinds of test on some 

 definite system based on a knowledge of the forces or in- 

 fluences which observation teaches us are the most effec- 

 tive agents in the results produced, we are simply the victims 

 of an old-world illusion that may serve to amuse, but cannot 

 instruct, those who indulge in it. 



The annexed figure (fig. 2) must, for present purposes, 

 be looked at without reference 

 to the rectangular plates seen Fig- 2. 



imbedded at its centre. As 

 a matter of fact, this par- 

 ticular sketch was made to 

 illustrate a point in connex- 

 ion with Dijffiugia symmetrica^ 

 of which mention will be made 

 presently. Apart, therefore, 

 from the presence of those 

 plates and a slight deviation 

 from the original outline, which 

 is by no means uncommon, 

 the figure might represent a 

 varietal form either of -D. mitri- 

 formis or of D. pyriformis. 



