514 



MOLLUSCOUS ANIMALS GENERALLY. 



Fig. 261. 



trumpet-shaped; and it is usually narrowed rather suddenly, 



when, as sometimes happens, a 

 new internal layer is formed as 

 a lining to the preceding (Fig. 

 261, A, d d). Hence the diame- 

 ter of these canals, as shown in 

 different transverse sections of 

 one and the same shell, will 

 vary according to the part of its 

 thickness which the section hap- 

 pens to traverse. The different 

 species of Terebratulidce, how- 

 ever, present very striking diver- 

 sities in the size and closeness 

 of the canals, as shown by sec- 

 tions taken in corresponding 

 parts of their shells; three ex- 

 amples of this kind are given 

 for the sake of comparison in 

 Figs. 262-264. These canals 

 are occupied, in the living state, 

 by tubular prolongations of the mantle, the interior of which 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 csecal tubes may be inferi'ed to 



Vertical seclioTis of Shell of rere6ro(itZtt (Wald- 

 heitnia) australls : — showing at A the canals 

 opening; by large trumpet-shaped orifices on the 

 outer surface, and contraclinp at d, d, into nar- 

 row tubes; and presenting at B a bifurcation of 

 the canals. 



Fig. QCa. 





Fig. 263. 



FiQ. 26-1. 





'auk 



Fig. 262. Horizontal section of Shell of TtnhraMa bullata (fossil, oolite). 

 Fig. 263. ' ' ' 



Fig. 264. 



of Megerlia lima (fossil, chalk), 

 of Spiriferina rostrata (triassic). 



possess a respiratory function ; and seem to be analogous to tubes 

 of a very similar nature, which extend into the "test" of many 

 Tunicata from their sinus system (§ 334). In the family Bhyncho- 

 nelUdce, which is represented by only two recent species (the Bh. 

 psittacea and Rh. nigricans, both of which formerly ranked as 

 Terebratulfe), but which contains a very large proportion of fossil 

 Brachiopods, these canals are entirely absent ; so that the unifor- 

 mity of their presence in the Terebratulidse, and of their absence 

 in the Rhynchonellidse, supplies a character of great value in the 



