808 MICROSCOPIC FORMS OF ANIMAL LIFE 



the exterior, save through these marginal pores, being nourished 

 by the transmission of the products of digestion from zone to zone 

 through similar bands of protoplasmic substance. In all cases in 

 which the growth of the disc takes place with normal regularity it 

 is probable that a complete circular zone is added at once. Thus 

 we find this simple type of organisation giving origin to fabrics of 

 by no means microscopic dimensions, in which, however, there is no 

 other differentiation of parts than that concerned in the formation 

 of the shell, every segment and every stolon (with the exception of 

 the two forming the ' nucleus ') being, so far as can be ascertained, 

 a precise repetition of every other, and the segments of the nucleus 

 differing from the rest in nothing else than their form. The equality 

 of the endowments of the segments is shown by the fact of which 

 accident has repeatedly furnished proof that a small portion of a 

 disc, entirely separated from the remainder, will not only continue 



FIG. 611. Disc of Orbitolites italica, Costa, sp. (O. tenuissima, Carp.), 

 formed round fragment of previous disc. 



to live, but will so increase as to form a new disc (fig. 611), the want 

 of the ' nucleus ' not appearing to be of the slightest consequence, 

 from the time that active life is established in the outer zones. 



One of the most curious features in the history of this type is its 

 capacity for developing itself into a form which, whilst funda- 

 mentally the same as that previously described, is very much more 

 complex. Tn all the larger specimens of Orbitolites we observe that 

 the marginal pores, instead of constituting but a single row, form 

 many rows one above another ; and, besides this, the chamberlets 

 of the two surfaces, instead of being rounded or ovate in form, are 

 usually oblong and straight-sided, their long diameters lying in a 

 radial direction, like those of the cyclical type of Orbiculina. When 

 a vertical section is made through such a disc, it is found that these 

 oblong chambers constitute two superficial layers, between which 



