54 



OUTLINES OF NATURAL HISTORY. 



CHAPTER XV. 



CLASS BRACHIOPODA. 



This class comprises a number of shell-fish which agree 

 with one another in having the body enclosed within a 

 double ("bivalve") shell, and in having two long spiral 

 processes, or " arms," attached to the sides of the mouth. 

 From this last-mentioned peculiarity the name of the class 

 is derived (Greek, hrachion, arm ; ^joc^es, feet). Owing to 

 the great rarity of the few forms of Brachiopods which 

 inhabit British seas, a foreign representative of the group 

 has been selected — viz., Terebratula flavescens of the seas of 

 Australia ; and a brief description will suffice to indicate 

 its leading peculiarities. 



The body of Terebratula flavescens is soft, and the 

 internal organs are enclosed in a modification of the in- 

 tegument, which is termed the " mantle." The front and 

 back portions of the mantle produce a shell, which is 

 composed of carbonate of lime, and conceals the animal 

 within it. The shell (fig. 22, A) is double, or, in other 

 words, consists of two distinct pieces, which are called 

 " valves," so that the shell is said to be " bivalve " (Latin, 

 his, twice; valval, folding-doors). The entire shell has 

 somewhat of the shape of an antique Roman lamp, each 

 valve having a prominence or " beak " at one extremity, 

 and one of them having this beak perforated by a round 

 aperture (/), similar to the hole through which the wick 

 passes into the lamp. (Hence the Brackiopoda are some- 

 times called " lamp-shells.") 



The two valves of the shell are jointed together at the 

 "beaks" by means of interlocking teeth and sockets, 

 and they are of decidedly different sizes. The smaller 

 valve lies over the back of the animal, and is not perfor- 

 ated by any aperture. Internally, however, it carries a 

 singular shelly loop (fig. 22, B, I), which supports tlie 



