MOLLUSCOIDA I BRACHIOPODA. 



313 



Mollusca (Lamellibranchiatd), 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 Brachio- 

 poda is usually the largest, and usually possesses a prominent 

 curved beak. The beak (fig. 126) is sometimes perforated by 

 a "foramen," or terminal aperture, through which there is trans- 

 mitted a muscular peduncle, whereby the shell 

 is attached to some foreign object. In some 

 cases, however (as in Lingula, fig. 127), the 

 peduncle simply passes between the apices of 

 the valves, and there is no foramen ; whilst in 

 others (as in Crania) the shell is merely at- 

 tached 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 

 1 2 with the surfaces of the shell " (Carpen- 

 ter). In most cases, also, the shell is perfo- 

 rated 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 widen- 

 ing as they approach the external surface'. 

 These canals give the shell a " punctated " 

 structure, and in the living animal they con- 

 tain caecal tubuli, or prolongations, from the 

 mantle, which are considered by Huxley as Fi s- I2 7- Lingula. 

 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 Rhynchondlida) 

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

 system. 



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

 expansions 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 

 of the shell, which is partitioned off by a membranous septum, 

 which is perforated by the aperture of the mouth. The re- 

 mainder of the cavity of the shell is almost filled by two long 



anatina, showing 

 the muscular ped- 

 uncle by which the 

 shell is attached. 



