56 ZOOLOGY SECT. 



increase in the number of chambers there is a multiplication of 

 the nucleus (Fig. 36, 4, b, nu). 



The shell presents two leading types of structure apart from 

 the form and arrangement of the chambers : either it is of a dense 

 porcelain-like texture and provided with a single terminal aperture 

 (imperforale, Fig. 36, 4), or the texture is more open and the whole 

 shell is perforated with very minute apertures, through which, as 

 well as through the terminal aperture, pseudopods are protruded 

 (perforate, Fig. 36 ; 2). 



In many cases additional complexity is attained by the develop- 

 ment of an elaborate canal system in the more complicated perforate 

 forms, and, in certain cases, of what is called the supplemental skeleton 

 (Fig. 37, 8b, s. sJc.). This consists of a deposit of calcium carbonate 

 outside the original shell ; it is traversed by a complex system of 

 canals containing protoplasm and is sometimes produced into large 

 spines. Foraminifera in which this secondary skeleton occurs are 

 sometimes of considerable size 2-3 cm. in diameter and of 

 extraordinary complexity. 



Many Foraminifera resemble Difflugia in having a skeleton 

 formed of sand-grains, sponge-spicules, and other foreign bodies 

 cemented together by a secretion from the protoplasm (Fig. 37, 1). 

 Some of these are formed on the imperforate type, having the 

 protoplasm protruded from a single terminal aperture ; others on 

 the perforate type, small pseudopods being protruded between the 

 particles forming the shell. 



In many cases the pseudopods are the only portions of proto- 

 plasm outside the shell, whereas in Gromia, as we saw, the shell 

 is invested with a layer of protoplasm, and is thus in strictness 

 an internal structure. A similar layer invests the surface in the 

 calcareous forms with perforate shells and gives off pseudopodia in 

 groups. In one of the calcareous forms with perforated spiral 

 shell, called Hastigerina (Fig. 38), a very remarkable modification of 

 this condition of things obtains. The shell (sh.) is surrounded with 

 a mass of protoplasm (plsm.) many times its own diameter, and so 

 full of vacuoles as to present a bubbly or frothy appearance. The 

 shell itself, moreover, in this and allied forms is provided with 

 numerous delicate, hollow, calcareous spines (sp.), which are only 

 to be seen in perfect, freshly-caught specimens. 



Many Foraminifera exhibit the phenomenon of dimorphism : 

 the individuals of a single species occur under two distinct forms 

 (megaspheric and microspheric) differing from one another in the size 

 of the central chamber, the shape and mode of growth of the suc- 

 ceeding chambers, and the number and size of the nuclei (Fig. 39). 



The reproduction of Foraminifera is mainly by spore-formation, 

 with or without copulation. The protoplasm has been observed 

 in some to divide into minute masses which may be amoeboid or 

 may be of the nature of flagellulse each provided with a 



