272 BRACH10PODA. 



On opening the shell the structure of the enclosed 

 Mollusk is at once seen to differ widely from that of 

 the Scallop and all the Conchiferous class. On each 

 side of the mouth, which is placed at the bottom of 

 the fold of the mantle, extends a fleshy arm, fringed 

 with long cilia. In some species, these arms are of 

 great length, and can be protruded from the shells 

 to a considerable distance, or retracted into elegant 

 spiral folds at the pleasure of the animal. 



The most obvious function attributable to the tentacular arms is 

 that of procuring food ; for being otherwise deprived of prehensile 

 instruments, without some adequate mechanism these helpless crea- 

 tures, imprisoned in their shelly covering, and fixed in one locality, 

 would be utterly unable to obtain nourishment necessary for their 

 support. The contrivance for this purpose is found in the arms, 

 which, covered by cilia, produce powerful currents in the surrounding 

 water, and these, being directed towards the mouth as to a focus, 



FlO. 209. SHELLY FRAMEWORK OF BRACHTOPOD. 



hurry down the throat of the animal whatever nutritive particles 

 may happen to be in the neighbourhood. The muscles supplied for 

 closing the shell in the Conchifera are never more than two in number, 

 and these pass immediately from one valve to the other : in the Brachio- 

 pods, on the contrary, the muscular system is very complicated, no fewer 

 than six pairs being provided either to act upon the valves or to 

 move the animal upon its pedicle. Their shells, moreover, contain 

 a complex framework for the support of the arms (Fig. 209). Seeing, 

 therefore, tli at these creatures differ from all other bivalves in almost 

 every part of their structure, there can be little doubt of the pro- 

 priety of considering them as forming a distinct class. 



