MOLLUSCA: BRACHIOPODA. 



389 



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 



Fig. 202. Rhynchonella suicata. A, Profile view. B, View ot the dorsal surface. 

 C, View of the base, a Ventral valve ; b Dorsal valve ; / Base ; c Beak ; h Foramen. 

 Lower Cretaceous. 



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 (figs. 

 202, 204) is often 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, fig. 203), the 

 peduncle simply passes between the apices of the valves, and 

 there is no foramen ; whilst in others (as in Crania, fig. 203, 

 D) the shell is merely attached by the substance of the ven- 

 tral 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). In most cases, also, 

 the shell is perforated 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 shells a "punctated" structure, and in the living animal they 

 contain caecal tubuli, or prolongations, from the mantle, which are con- 

 sidered 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 Rhynchonellidce) 

 the shell is "impunctate," or is devoid of this singular canal system. 



Though characteristically calcareous, the shell of the Brachiopoda may 

 sometimes be largely composed of horny matter (as in Discind) ; or the 

 carbonate of lime in the horny shell may be almost wholly replaced by 

 phosphate (as in Lingicla). 



