FORAMINIFERA. 



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FORAMINIFERA. 



little can with certainty be laid down re- 

 garding their structural and systematic rela- 

 tions. They are contained within calcareous 

 shells, which are polythalamous or divided 

 transversely by partitions or septa into a 

 number of chambers. The chambers in some 

 are perfectly distinct from each other, 

 although aggregated to form a compound 

 shell (PI. 18. figs. 17, 18) ; in others, the 

 septa are perforated with one or more aper- 

 tures (figs. 10, 19), the margins of which are 

 sometimes tubular or prolonged to form a 

 sipho or funnel-like tube (fig. 10). The 

 shell is variable in shape ; but it is usually 

 either cylindrical, consisting of a number of 

 cells or chambers arranged end to end in 

 one or more rows, or spiral, when the 

 rows of chambers are coiled. As the more 

 recently-formed chambers are often larger 

 than the others, the shells are generally more 

 or less pyramidal. In a few genera and 

 species, the shell consists of a single cell 

 only, without septa. 



The outer surface of the shells presents a 

 punctate or dotted appearance, arising from 

 the presence of very numerous foramina, 

 whence the name Foraminifera. The fora- 

 mina are the outer orifices of tubes passing 

 through the walls of the shell. The arrange- 

 ment of these tubes and that of another set, 

 traversing the walls and the septa of the 

 shells, as well as, in fact, the general struc- 

 ture of the shell, may perhaps be well 

 illustrated by a description of the shell of 

 Operculina arabica (PI. 18. fig. 22), in which 

 they have been carefully traced by Mr. Carter. 

 In the shell of Operculina arabica, the outer 

 surface, after the removal of a greenish epi- 

 dermic layer, is seen to be covered with large 

 and small papillae (fig. 23) ; the former (b) 

 1-2150", the latter (a) 1-8600" in diameter, 

 neither of which are present over the septa 

 or at the margin of the shell. Each of the 

 septa encloses within its walls two calcareous 

 tubes, spaces or vessels, one on each side 

 the intraseptal vessels (figs. 26 a, 27 c); these 

 are about 1-1900" in diameter, and in their 

 course give off two sets of lateral branches, 

 terminating upon the two surfaces of the 

 septum in which they run. The tubes 

 communicate at each end with a network of 

 smaller ones; one set of which ramifies in 

 the upper, the other in the under wall or 

 margin of each chamber ; these are the mar- 

 ginal plexuses (fig. 27 / /), and the former 

 terminate upon the outer margin of the shell 

 (9 9}- The inner wall of the chambers is 

 pierced by innumerable tubes about 1-9000" 



in diameter, which pass directly downwards 

 from the small papillae on the outer surface 

 (fig. 25 d). In a vertical section of the shell, 

 in addition to these tubes, seven, eight or 

 more parallel horizontal lines are seen (fig. 

 25 c) ; these are the lines of contact of the 

 layers composing the shell, or the lines of 

 growth. The margin of the shell is composed 

 of hollow calcareous spicula (fig. 24), 1 -23/" 

 long and 1-900" broad. In a transverse sec- 

 tion of the margin, more than 100 of these 

 are seen, forming a triangular bundle or 

 cord (fig. 28 a), the apex being directed 

 towards the chamber, the base outwards 

 forming the free rounded margin of the 

 shell, and parallel to its sides run the papil- 

 lary tubes of the chamber (fig. 28 b). 



The tubes passing through the walls of 

 the chambers are common to all the Forami- 

 nifera, but the papillae are not ; whether the 

 spicula, the intraseptal vessels and the mar- 

 ginal plexuses are of general occurrence, has 

 not been ascertained; but the two latter 

 have been detected by Mr. Williamson in the 

 shell of Faujasina, who denies, however, 

 their communication with the chambers, 

 admitting merely that with the common 

 tubes of the shell. 



In addition to the common foramina and 

 the orifices of the marginal plexus, the 

 chambers, especially those which terminate 

 the series, are furnished with other larger aper- 

 tures opening externally; these are of various 

 forms and differently situated; sometimes 

 they are round, numerous and comparatively 

 small (PI. 18. fig. 13); at others they are 

 single and large (fig. 18), circular, semicir- 

 cular or lunate (fig. 3). According toEhren- 

 berg, these are often closed by a valve, 

 so as to be invisible when the animal is con- 

 tracted or dead. This author also compares 

 them with the apertures of the polype-cells 

 of Pennatula, Alcyonium and Lobularia, in 

 which calcareous particles contained in the 

 integument so completely close the cells, 

 as to render the opening no longer percep- 

 tible. The reader will not fail to perceive 

 the analogy between the closure of the cells 

 in the latter instances by calcareous particles, 

 and in Operculina by the cord (?) of calca- 

 reous spicula. But how much of the evidence 

 upon which Ehrenberg's views upon the 

 Foraminifera are based, rests upon analogical 

 reasoning, and how much upon observation, 

 is known only to himself, as is unfortunately 

 too often the case in regard to his results. 



The nature of the animal body contained 

 within the shell has been very differently 



