RHIZOPODA. 9 



diameter of 5 /x.. The intervening stages between the zoospore, produced by the 

 megalospheric form, and the microsphere in which the microspheric form takes its 

 origin, have not been followed ; but there is some, though at present incon- 

 clusive evidence, in favour of the supposition that the microsphere results 

 from the conjugation of two zoospores. The relative sizes of the mici^sphere 

 and zoospore 10 /*. and 5 p., agrees fairly well with this view. One of the 

 earliest observations relating to the dimorphism of the Foraminifera was of 

 the well-established fact that the microspheric form is much less abundant 

 than the megalospheric, and this admits of easy explanation on the supposition 

 that the union of two separate organisms is required for its production. Finally, 

 Schaudinn's observation of the conjugation of the zoospores of Hyalopus, 

 which is, however, a form not known to be dimorphic, supports the hypothesis. 



From the foregoing account of the life-history of Polystomella it appears that 

 the dimorphism of the Foraminifera is due to the occurrence of alternating or 

 recurring generations, and there is some, though at present inconclusive 

 evidence in support of the view that the megalospheric generation arises 

 asexually, and the microspheric generation as the result of conjugation. In the 

 genus Orbitolites (Miliolidoe), while the megalospheric form has been found to 

 be produced from a microspheric form as in Polystomella, it has also been 

 seen to arise from a megalospheric parent. In this case, then, it must be 

 supposed that the generations do not regularly alternate, but that the megalo- 

 spheric form may be repeated before the brood of zoospores is produced. 



Besides the difference in the size of the initial chambers, the shells of the 

 two forms present in some cases marked differences in the mode of growth. 

 Thus in the genus ^Biloculina* (Miliolidce), while the mode of growth of the 

 megalospheric form is, as shown by Schlumberger, on the biloculine plan from the 

 first (Fig. 5), that of the microspheric form is at first on the qiiinqueloculine 

 plan, and it is not until many chambers have been formed that the biloculine 

 plan, characteristic of the genus, is assumed. 



The application of the term dimorphism to the phenomenon above 

 described is in accordance with its general use in zoology and 

 botany. It has, however, been used, together with the terms 

 trimorphism and polymwpliism, in another and quite different sense, 

 namely to indicate the occurrence of two (three, or more) different 

 modes of growth in the building up of the shell of a single 

 individual. Thus, in the shell of the genus Pener&plis, the chambers 



* The mode of growth of the shells of the genera Biloculina, Triloculina, 

 and Quinqueloculina is a modification of the spiral. The chambers are elongated 

 and increase in size as they succeed one another, each occupying half a turn of 

 the spiral. The result is that the mouths of the chambers are directed succes- 

 sively in opposite directions, and a long axis of the shell is thus established. 



In the genus Biloculina, (Fig. 5) the chambers lie in the same plane, and 

 each overlaps its predecessors at the sides. Hence only two chambers are 

 exposed in the outer contour of the shell. In the other genera the chambers 

 are narrower and do not lie in the same plane, the median plane of each being 

 directed at a definite angle, which is constant for the genus, to that of its 

 predecessor. The result is that in one case three, and in the other five 

 chambers are exposed in the contour of the shell, and the triloculine and 

 quinqueloculine forms of shell are respectively produced. The genei-a Triloculina 

 and Quinqueloculina (d'O.) were by Williamson included in the genus Miliolina. 



