602 



PERFOEATED SHELLS OF TEREBRATULID^E. 



(Fig. 312, a), whilst externally they are covered by the Periostra- 

 cuni (b). Their diameter is greatest towards the external surface, 

 where they sometimes expand suddenly, so as to become trumpet- 

 shaped ; and it is usually narrowed rather suddenly, when, as some- 

 times happens, a new internal layer is formed as a lining to the 

 preceding (Fig. 313, a, d d). Hence the diameter of these Canals, 



as shown in dif- 

 a Fig. 313. b ferent transverse 



sections of one and 

 the same Shell, 

 will vary accord- 

 ing to the part 

 of its thickness 

 which the section 

 happens to tra- 

 verse. The shells 

 of different species 

 of perforated Bra- 

 chiopods, how- 

 ever, present very 

 striking diversi- 

 ties in the size and 

 closeness of their 

 Canals, as shown 

 by sections taken 

 in corresponding 

 parts; three exam- 

 ples of this kind 

 are given for the 

 sake of compari- 

 son in Figs. 314-316. 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 be considered to be the animal's 

 Blood. Hence these caecal tubes may be inferred to possess a 

 Respiratory function. 



466. In the Family Fhynehonellidce, which is represented by 

 only two Recent species (the Rh. psittacea and Rh. nigricans, both 

 of which formerly ranked as Terebratulse), but which contains a 

 very large proportion of Fossil Brachiopods, these Canals are entirely 

 absent ; so that the uniformity of their presence in the Terebra- 

 tulidse, and of their absence in the Rhynchonellidse, supplies a 

 character of great value in the discrimination of the fossil shells 

 belonging to these two groups respectively. Great caution is 

 necessary, however, in applying this test ; mere surface-markings 

 cannot be relied-on ; and no statement on this point is worthy of 

 reliance, which is not based on a Microscopic examination of thin 



Vertical Sections of Shell of Terebratula (Wald- 

 heimia) australis : — showing at a the canals open- 

 ing by large trumpet-shaped orifices on the outer 

 surface, and contracting at d d into narrow tubes ; 

 and showing at B a bifurcation of the canals. 



