MOLLUSCOIDA : BRACHIOPODA. 245 



Mollusca (the Lamellibrancliiata), but their organisation is 

 very much inferior, and there are also sufficient differences in 

 the shell to justify their separation. 



The two valves of the shell of any Braehiopod are articulated 

 together by an apparatus of teeth and sockets, or are kept in 

 apposition by muscular action alone. One of the valves is 

 always slightly, sometimes greatly, larger than the other, so 

 that the shell is said to be ' inequivalve.' As regards the 

 contained animal, the position of the valves is anterior and 

 posterior, so that they are, therefore, termed respectively the 

 ' ventral ' and ' dorsal ' valves. In the ordinary bivalve 

 Mollusca (Lamellibranchiata), on the other hand, the two 

 valves of the shell are usually of the same size (equivalve), 

 and they are situated upon the sides of the animal, so that, 

 instead of being dorsal and ventral, they are now termed 

 ' right ' and * left ' valves. The ventral valve in the shell of 

 the Brachiopoda is usually the largest, and usually possesses 

 a prominent curved beak. The beak is sometimes perforated 

 by a 'foramen,' or terminal aperture, through which there is 

 transmitted a muscular peduncle, whereby the shell is attached 

 to some foreign object. In some cases, however (as in 

 Lingula),ihe peduncle simply passes between the apices of the 

 valves, and there is no foramen ; whilst in others (as in 

 Crania), the shell is merely attached by the substance of the 

 ventral valve. The dorsal, or smaller, valve is always free, 

 and is never perforated by a foramen. 



In intimate structure, the shell of most of the Brachiopoda 

 consists ' of flattened prisms, of considerable length, arranged 

 parallel to one another with great regularity, and at a very 

 acute angle usually only about 10 or 12 with the surfaces of 

 the shell.' (Carpenter.) Inmost cases, also, the shell is per- 

 forated by a series of minute canals, which pass from one 

 surface of the shell to the other, in a more or less vertical 

 direction, usually widening as they approach the external 

 surface. These canals give the shell a ' punctated ' structure, 

 and in the living animal they contain csecal tubuli, or pro- 

 longations, from the mantle, which are considered by Huxley 

 as analogous to the vascular processes by which in many 

 Ascidians the muscular tunic, or 'mantle,' is attached to the 

 outer tunic, or ' test.' In some of the Brachiopoda (as in the 

 Rkynchonellidce) the shell is ' impunctate,' or is devoid of this 

 singular canal system. 



The inner surface of the valves of the shell is lined by ex- 

 pansions of the integument which secrete the shell, and are 

 called the ' lobes ' of the ' pallium,' or ' mantle.' The diges- 

 tive organs and muscles occupy a small space near the beak 



