September, 1910. 



KNOWLEDGE. 



377 



no {jerforations in the shell wall. PolystonicUa 

 crispa, on the other hand, is a representative of the 

 hyaline or perforate group, which, in additjion to one 

 or more special apertures, have the shell wall 

 perforated with innumerable fine pores through 

 which the pseudopodia can be extruded. The 

 special aperture in the case of P. crispa consists 

 of a line of \'-shaped pores situated on the 

 face of the last chamber, close to the wall of the 

 preceding convolution. These pores, which are the 

 true " foramina," in the original and strict sense of 

 the word, form the principal channel for the passage 

 of the protoplasm from the animal, hut in addition 

 there are countless numbers of extremeh' minute 



marked by the growth of strong spines, which are 

 prolongations of the septal ridges. These spines 

 decrease in size and number as the shell increases in 

 size, but they can often be traced in specimens of 

 considerable age. A full-grown specimen averages 

 one-fifteenth of an inch in diameter. 



Owing to the abundance of PoIystoiiicHa crispa 

 and its readiness to adapt itself to ' an artificial 

 existence, it has formed a favourite study for the life- 

 history of the group with many specialists. The 

 result is that we know its life-history with greater 

 completeness than that of any other Foraminifer, 

 principally owing to the exhaustive researches of 

 Mr. T. T. Lister, F.R.S. Much of what we now 





A. Megalospheric form. 



B. Microspheric form. 



Figure 2. 



PolystomeUa crispa decalsified to show the sarcode. b. the central chambers of the microspheric form more highly 

 magnified. The canal s}-stem is omitted. Nuclei deeply stained. ;- = retral processes, si. = stolon or communication between 



chambers.'' 



tubuli through the shell substance, which are so fine 

 as to be visible only bv transmitted light and under 

 a fairly high power of the microscope. 



The external surface of the shell is covered with a 

 series of raised bands, radiating from the central 

 umbo in a series of graceful curves. These are the 

 septal ridges, and mark the divisions between the 

 separate chambers of the shell. Projecting back- 

 wards between the septal ridges, and at right angles 

 to them, are a series of smaller ridges marking the 

 surface of the wall of each chamber. These are the 

 coverings of the so-called " retral processes," which 

 are pockets in the shell substance into which 

 prolongations of the protoplasmic bod\- extend. The 

 " retral processes " are a characteristic feature of the 

 genus PolystomeUa. 



The peripheral edge of the shell may be more or 

 less rounded, but it is generally produced into a 

 sharp keel, and often marked by a deposit of clear 

 shell substance. \n verv \'oung shells the points of 

 intersection of the septal ridges and the keel are 



know as to the curious ''dimorphism," long 

 recognised in the Foraminfera, is based on his 

 discoveries in connection with PolystomeUa crispa. 



First, let us give a brief account of dimorphism. 

 It was very early recognised, from the study of 

 Nummulites, that in a series of specimens from one 

 gathering, the majority were of one fairly uniform 

 size, while a small minority were of much larger 

 dimensions. These two types, although occurring in 

 the same deposit, were generall\- regarded as separate 

 species, and many theories were formed to account 

 for their constant and mysterious association. 

 Closer examination of the specimens showed that 

 the small specimens commenced their existence with 

 a large primordial or initial chamber, and that this 

 was followed by relatively few convolutions of 

 chambers. The large specimens on the other hand 

 were found to commence life with a small primordial 

 chamber followed by many convolutions. The type 

 with the large primordial chamber is called the 

 me^alosplieric form or Form A, while those with the 



'■'Figures 2, 3 and 4 are reproduced by kind permission of Mr. J. J. Lister, F.K.S., from his monograph on the Foraminifera in 



Lankester's Treatise on Zoology. 



