304 M. F. Drejer 07i ike 



a primary axis is not the only thing, but their fundamental 

 form undergoes further differentiations. In tlie first phice a 

 difference of the transverse axes makes itself felt in such a 

 way tliat a long and a short transverse axis may be recog- 

 nized, these being perpendicular to each other and to tlie 

 principal axis. The result represents the fundamental form 

 of the amphitect pyramid (Hackel) ; the forms belonging here 

 are lenticularly flattened laterally, «'. e. parallel to the primary 

 axis. In such Rhizopod shells, moreover, the pylom is fre- 

 quently no longer round, but drawn out in the form of a slit ; 

 any spines present at the aboral pole are generally orientated 

 in the direction of the longer cross-axis ; sometimes the peri- 

 phery of the monaxon-lenticular shell is keeled. Such more 

 or less distinctly amphitect-pyramidal promorphs occur in 

 JEi/alosjj/iem'a, Quadrula^ Diffiugia, Euglypha, Gromia^ 

 Lagena [Fissurina, Rss.), and Lingulina, in some pylomatic 

 Spumellaria *, in various Nassellaria, and throughout in the 

 Phajodarian family Challengerida. 



A further step towards higher differentiation is the tran- 

 sition to the eudipleural (bilaterally symmetrical) fundamental 

 form, which may start either from amphitect or from simply 

 monaxonic forms. This takes place in general in conse- 

 quence of an elongation of the pylora (which in monaxonic 

 and amphitect Rhizopoda is situated at one pole of the primary 

 axis, and, indeed, directly perpendicularly beneath the apical 

 pole of the shell) forward or backward, by which means a 

 front and back and right and left become distinguishable. It 

 is interesting to mark the agreement of this process with the 

 transformation of the primary form in the hypothetical deve- 

 lopment of the Turbellaria from Ctenophora (A. Lang). The 

 Ctenophora and earliest Turbellaria are perfectly amphitect in 

 structure ; the mouth is placed in the middle of the underside 

 perpendicularly beneath the apical pole of the body, front and 

 back, right and left are not yet distinguishable, and this dis- 

 tinction is only produced by elongation of the mouth forward 

 or backward, which occurs in most Turbellaria (Polyclada), 

 and by which the eudipleural fundamental form is given. 

 Moreover, many Rhizopoda become eudipleural by a corre- 

 sponding arrangement of the oral and aboral radial appendi- 

 cular structures or by a bending round of the apertural neck 

 of the shell. Eudipleural development of the shell occurs in 

 Diffiugia^ Trinemciy Gyphoderia, Campascus^ Lieherkuknia^ 

 Microgromiaj Flatoum, P/ectophrgs, and Lecgfhium, in many 



* The pylomatic Discoidea and Larcoidea are, however, to be excepted 

 from this series of phenomena. For further details upon this point see 

 my ' Kadiolarifnstudien,' Heft i. pp. 98, 99. 



