6 AXKL GOES, ARCTIC AND SCANDINAVIAN FOBAMTNIFERA. 



If we examine, for instance, a "Flabelline" Frondicularia, Fig. I, A, we will readily 

 distinguish a small embryonal or primordial segment (e) that brings forth a larval stage with 

 five segments (/), disposed in a manner which constitutes the Vaginulina shape or the nearly 

 allied Crepidula form amongst the Nodosarina?. Subsequent to this stage we find the seg- 

 ments arranged in the peculiar Frondicularia manner, representing the mature or fully de- 

 veloped stadium («?), and commencing when the larval stage has completed its fifth segment. 



Another modification of the same form is represented in Fig. B. The initial seg- 

 ment is here about twice as big as in A, or somewhat more, and the larval stage shows 

 but three segments, whereupon the mature Frondicularia stage ensues. 



Lastly we meet with the form C, generally provided with a larger initial segment, 

 which regarding its power of growth or maturity seems to equalize the whole embryonal 

 and larval condition of the animal, assuming at once the mature Frondicularia segments. 

 In all other respects these three forms resemble each other and ought not be ranked as 

 different species. They have all originated from somewhat differentiated primordial embryo- 



e e 



Y\s. I. Frondicularia alata d'Okb. with 3 difl'ereiit forms of its larval stage.' 



segments, and the arrangement of the subsequent segments apparently bears a certain 

 physiological relation to the development of the initial stage. 



The least developed primordial segment (7) is apparently tending to bring foi'th a 

 larval stage that inherits the form of the original type (here Vaginulina) and it has to go 

 through a more retarded evolution before it reaches maturity (fig. I A). A more powerful 

 initial segment (^) gives rise to a more readily performed larval development, its seg- 

 ments being stronger but reduced in number, and more apt to grow into the mature 

 stage (fig. I jB). The highest developed embryo-segment (3) gives rise to a true "Frondi- 

 cularia" (Fig. C) without any intervenient larval stage. Thus it will be obvious that the 

 number and form of the larval segments must become subject to great variation, and 

 stages with two and several segments will therefore be met with in the same species of 

 FrondiculariiP. 



Thus, in this instance, we have before us tlie plainest proceeding of evolution from 

 one type to another, in which an earlier type becomes larva for another type. At last 

 the larval condition is reduced to a single segment and a new form has originated, seem- 

 ingly standing without much morphological connection with its origin. 



' The different embryo seg;raents are represented as occupants of the mature segments, but in reality the 

 larger ones are probably generated outside of the chambers. 



