276 PHYSIOLOGY OF THE INVERTEBRATA. 



by the deposit of calcareous salts, sometimes containing a 

 large proportion of phosphate of lime {Lingula)." 



In Waldheimia and other Brachiopods, "the perivisceral 

 cavity communicates with the pallial chamber by at least two, 

 and sometimes four, tubular organs, which have been described 

 as hearts, but are now known to have no such nature." 



These organs are funnel-shaped, the wide parts of which 

 open into the perivisceral cavity. The narrower parts of these 

 organs pass through the anterior wall of the visceral chamber, 

 and terminate in small openings in the pallial cavity. 



According to Dr. Morse, the ova pass through these organs 

 in Terebratulina septentrionalis. The so-called pseudo-hearts 

 have a double function, being renal organs and genital ducts. 

 They are the homologues of the organs of Bojanus of the 

 Mollusca, and of the segmental organs of worms. 



The Mollusca. 



The excretion of carbonate of lime is an important function 

 in a large number of Molluscs. 



In Anodonta, which is taken as a typical example of the 

 Zamellihranchiata, the shell is a " cuticular excretion from 

 the surface of the mantle," and consists of variously disposed 

 lamellae of organic matter impregnated and hardened by the . 

 deposition of calcareous salts (chiefly carbonate of lime, 

 mineralised as arragonite). The shell has no cellular struc- 

 ture ; "but from the disposition of its lamellse, and from the 

 manner in which the calcareous deposit takes place in them, 

 it may present varieties of structure which have been distin- 

 guished as nacreus, prismatic, and epidermic " (Fig. 55). 



In the young Lamellibranch shell there is a much larger 

 percentage of calcium phosphate present than in the adult 

 shell: the calcium phosphate being gradually replaced by 

 calcium carbonate as the animal arrives at maturity. 



The ligament which unites the valves together is an uncal- 

 cified chitinous material. This material is continuous with 



