468 RECENT BRACHIOPODA chap. 



Spirifera the internal skeleton takes the form of two spirally 

 coiled lamellae, which almost entirely fill the cavity of the shell ; 

 the apices of the spirals point outwards (Fig. 330). The inner 

 surface of the shell also bears the marks of the insertion of the 

 numerous muscles which govern its movements. 



Microscopic examination of thin sections of the shell shows 

 that it consists of small prisms or spicules of calcareous sub- 

 stance, whose long axis lies, roughly speaking, at right angles to 

 the surface of the shell. These spicules are held together by 

 an organic matrix, in which, however, no cellular elements can 

 be detected. In sections made through a decalcified shell the 

 position of the spicules which have been dissolved by the acid is 

 indicated by spaces, and the matrix remains as a network of fibrils, 

 which end on the outside in a thin cuticular layer of organic 

 matter. In Lingula and JDiscina the organic matter takes a 

 much larger share in the formation of the shell, which in these 

 genera consists of a number of alternating layers of horny and 

 calcareous matter. The former is described by Gratiolet as 

 fibrillated, the fibrils being obliquely placed, whilst the latter 

 consists of a number of small prisms set at right angles to the 

 surface of the shell. 



In many genera, as in Terehratula, Terebratella, Cistella, 

 Waldheimia, Crania, etc., the shell is pierced by a number of 

 small canals (Fig. 314), which in the dried specimens form so 

 many open pores, but in the living animal contain prolongations 

 of the mantle or body wall which secretes the shell. They con- 

 tain extensions of the layer of epithelial cells which lines and 

 secretes the shell. The canals come to the surface and at 

 their outer end are often slightly swollen. They are closed by 

 the cuticular layer which is mentioned above as covering the shell 

 externally. They are not found in the loops or other internal 

 processes of the shell. In Crania the canals depart to some 

 extent from the usual type ; instead of running a straight course 

 to a somewhat swollen outer end, they break up into a number 

 of very fine branching tubules, which form a very minute mesh- 

 work near the surface of the shell. These fine branches con- 

 tain protoplasmic fibrils, which have their origin in the epithelial 

 cells which lie in the tubules. 



By carefully counting the number of tubules in a given area 

 of young and old specimens of Waldheimia cranium, van Bem- 



