70 



THE INVERTEBRATA 



grow by the addition of chambers. The protoplasm bulges from the 

 mouth of the shell and there secretes around itself a new chamber into 

 which opens the previous mouth. The chambers may be arranged in 

 a straight line, as in Nodosaria (Fig. 6 B), or in a spiral, as in 

 Polystomella, etc. (Figs. 62, 63, 65), or occasionally irregularly; and 

 the shell may be strengthened by the deposition, upon their original 



4 J3 



Fig. 62. A, Section of a foraminifer in which each septum is formed of 

 a single lamella. B, One in which the septum is formed of two lamellae and 

 a supplemental layer is present. After Carpenter, a, passages between the 

 chambers; b, septum; c, anterior wall of last chamber; d^ supplemental 

 skeleton. 



B 



Fig. 63. Dimorphism of Nummulites laevigatas, Bracklesham Beds (Eocene), 

 Selsea. From Woods. A, Section of the entire shell of the megalospheric 

 form, X 9. B, Section of the central part of the microspheric form, x 9. 



walls, of a supplemental layer (Fig. 62 B). The nuclei, where there 

 is more than one, bear no constant relation to the chambers. 



In many species the shells 2Lre dimorphic, the two forms (Figs. 63,66) 

 being distinguished by the size and arrangement of the first formed 

 chamber, which is small in one (the microspheric form) and larger in 

 the other (megalospheric). These forms correspond to the alternation 



