20 PROTOZOA— RHIZOPODA phylum i 



from the aperture and a new Chamber wall then formed about it. In some 

 cases a complete wall is formed with each newly added Chamber, but in others 

 the adjacent parts of previous Chambers form the inner walls of the new^ 

 Chamber, and new walls are formed only on the free i)arts of the protoplasmic 

 mass. In the open tubulär test, such as Astrorhiza or Hyperammina, increase 

 in the protoplasmic body is accompanied by addition of material at the open 

 end of the tube and an increase in size results. In single-chambered types, 

 such as Lagena, the manner of increase in size is problematical, if there be any 

 at all. In such forms the entire test may be made in its completed form at 

 once after division, as is the case in certain of the fresh-water Rhizopods. 



In the tests having more than a single Chamber the apertures of the first- 

 formed Chambers become internal as a rule, and a complexity of relations to 

 the outside medium is thus brought about. One of the simplest arrangements 

 of the Chambers is a linear series. Such an arrangement is seen in Ileophax 

 and Hormosina. Another very common plan of arrangement is a planospiral, 

 as in Ammodiscus. This may be varied by having the revolving line in a spire 

 and then the whole test becomes trochoid, as in Trochammina. Another 

 common arrangement is a biserial one, the Chambers being on opposite sides 

 of the axis, as in Textularia. These four plans or some modification of them 

 are the characteristic arrangements for the Chambers in most of the secreted 

 tests. Oftentimes more than one plan of arrangement enters into the forma- 

 tion of the test. Dimorphism was used for this, but that term has been used 

 elsewhere with a very different meaning. As here viewed, this life-history 

 with several distinct methods of growth has a deeper significance than has 

 usually been attached to it. It seems to have a definite phylogenetic bearing 

 in each particalar group. The term " dimorphism " would hardly cover the 

 case in some genera, where eight or more distinct stages may be made out, 

 each with its characteristic form of Chamber, yet all appearing successively in 

 a Single test. 



The number of Chambers in the complex tests varies from a few to a great 

 many. Where the size of the test becomes considerable and the Chambers 

 correspondingly large, the Chamber is often divided up in various ways into 

 chamberlets, as in Orbitolites. In such cases the adjoining chamberlets are 

 usually in free communication with one another. The walls of the chamber- 

 lets give additional strength in many forms in which they are developed. 

 Another characteristic modification in some genera is the development of 

 labyrinthic structures in the interior of the Chambers. Such structures are 

 seen in Oyclammina, Haplostiche, Fabularia, etc. In general, it seems tq be a 

 mark of the culmination of certain lines in development, and many of the 

 genera which developed such labyrinthic structures are now extinct. From 

 the appearance of a series of such tests of one species at different stages in 

 development, it would seem as though this labyrinthic condition was developed 

 as a secondary growth in the Chamber. One of its uses may be to give added 

 strength to the test, but this does not always seem to be the case, for it may 

 occur in tests which are characterised by thick walls. 



The aperture in a given species seems to be rather constant when the 

 development is understood. Much has been written upon this subject ; 

 apertural characters have been used by some authors as a basis for systematic 

 work, and discarded by others as very variable. In a few specimens it may 

 seem at first sight as though the apertural characters were very variable, but 



