176 MOLLUSCA— CLASS II. BRACHIOPODA. 



bling the branchiae of the preceding class in some species, that they were 

 at one time thought to be the true organs of respiration. These retractile 

 arms are said to be in constant activity for the purpose of producing an 

 inward current of water for the capture of animalculse and other alimen- 

 tary prey. Thirdly, in the arrangement and position of the branchiae : 

 instead of the organs of respiration being distinctly formed in lateral 

 lamella? upon the body as in many of the Tropiopoda, they consist of a 

 number of beautiful veins and arteries incorporated within the substance 

 of the two lobes of the mantle. The calcifying organ of the Brachiopoda, 

 therefore, has a double function : in addition to its usual property of 

 secreting the calcareous mucus for the formation of the shell, it is made 

 subservient to the circulation of the aerated fluid. Professor Owen 

 observes, " That in this profuse distribution of vessels over a plain 

 membranaceous surface, we perceive the simplest construction of the 

 water-breathing organ, presenting a beautiful analogy with the elementary 

 forms of the air-breathing organ in the Pulmoniferous Gasteropoda (He- 

 lices, Bulimi, e.g.*). The muscular system in these animals appears to 

 be most complex ; the Lingulce and Orbiculee are provided with three 

 pairs of muscles, and the Terebratula have four. The large muscles are 

 destined to open and close the valves in the absence of a hinge ligament, 

 and the small ones assist in sliding one valve over the other for the 

 admission of water. 



With regard to the situation that the Brachiopodous Mollusca should 

 occupy in the natural system, as well as the rank to which they are 

 entitled in the classification, authors have been much divided. By 

 Dumeril and De Roissy they were associated in a particular class with the 

 Lepades, on account of a fancied resemblance in their spirally twisted 



* In consequence of this new and peculiar arrangement of the respiratory system, the title 

 of Brachiopoda has been changed by De Blainville to that of Palliobrunchiata or mantle-breathing 

 mollusks. It is, perhaps, the more appropriate ; but we have been guided in our selection of 

 the former from a desire of basing the primary distribution of the Mollusca upon the organ of 

 movement throughout (poda, a irovs pes) ; and in substituting the term Tropiopoda for that of 

 Conchifera, in reference to the preceding class, we have selected the particular form of the 

 foot as best calculated to secure a precision of rank in the nomenclature. 



