MOLLUSCOUS ANIMALS GENERALLY. 



177 



ence, that the successive layers have but a comparatively slight adhesion 

 to each other. 



570. The shells of Tertibratula, however, and of most other Brachio- 

 pods, are distinguished by peculiarities of structure which differentiate 

 them from all others. When thin sections of them are microscopically 

 examined, they exhibit the appearance of long flattened prisms (Fig. 392, 

 A, b), which are arranged 



with such obliquity that FIG. 392, 



their rounded extremities A B 



crop-out upon the inner 

 surface of the shell in an 

 imbricated (tile-like) man- 

 ner (a). All true Terebra- 

 tulidce, both recent and 

 fossil, exhibit another very 

 remarkable peculiarity; 

 namely, the perforation of 

 the shell by a large number 

 of canals, which generally 

 pass nearly perpendicularly 

 from one surface to the other 

 (as is shown vertical sections, 



TTirr 3Q3\ anrl forTninnfp A Internal surface (a), and oblique section (6), of Shell 

 *lg. 6V6), ana terminate Qf Terebratlda (Wa ldheimia) ausfralis- B, external sur- 



mternally by open orifices face of the same. 

 (Fig. 392, A), whilst exter- 

 nally they are covered by the FIG. 393. 

 periostracum (B). Their dia- 

 meter is greatest towards 

 the external surface, where 

 they sometimes expand sud- 

 denly, so as to become trum- 

 pet-shaped; and it is usually 

 narrowed rather suddenly, 

 when, as sometimes hap- 

 pens, a new internal layer is 

 formed as a lining to the 

 preceding (Fig. 393, A,dd). 

 Hence the diameter of these 

 canals, as shown in different 

 transverse sections of one 

 and the same shell, will vary 

 according to the part of Vertical Sectiong of ghell of Terebratula 



Its tniCKlieSS Which the Sec- heimia) australis; showing at A the canals opening by 



firm harmpna fr> trnvprap large trumpet- shaped orifices on the outer surface, and 



nappens tO tl avei Se. contracting at d, d, into narrow tubes; and showing at B a 



1 The Shells Of different Species bifurcation of the canals. 



of perforated Brachiopods, 



however, present very striking diversities in the size and closeness of their 

 canals, as shown by sections taken in corresponding parts; three examples 

 of this kind are given for the sake of comparison in Figs. 394-396. 

 These canals are occupied in the living state by tubular prolongations of 

 the mantle, whose interior is filled with a fluid containing minute cells 

 and granules, which, from its corresponding in appearance with the fluid 

 contained in the great sinuses of the mantle, may perhaps be considered 

 to be the animal's blood. Of their special function in the economy of 



