MANUAL OF NATURAL HISTORY. 161 



phagus ; their organs of taste are probably situated 

 in the filaments of the mantle-margin ; and those of 

 touch most probably consist of the curious ciliated, 

 twisted or spiral, tentacular arms. Their digestive 

 organs do not differ materially from those of the 

 Bivalves, the alimentary canal commencing in a 

 simple oral aperture situated between the bases of 

 the tentacular arms ; in Terebratula it is tubular 

 and curved for some distance, and then becomes di- 

 lated into a stomach, while in other genera, it makes 

 several turns and continues throughout of the same 

 calibre ; there are no salivary glands, and the dis- 

 integrated liver pours its secretion directly into the 

 digestive tube. The structure of the shell varies 

 in different genera ; in Discina it is almost entirely 

 horny, in Lingula it is covered with an epidermis, 

 which in Terebratula is entirely wanting. The 

 shells of the Terebratulidce are finely perforated, 

 the tubular apertures being lined, in the living ani- 

 mal, with prolongations of the mantle. The Brachio- 

 pods are extremely numerous in fossil genera and 

 species, and appear to have been among the forms 

 of Mollusca earliest created. The recent genera are 

 few, and live in all seas, usually at very consider- 

 able depths ; the Graniidce being sessile on stones 

 and other sub-marine bodies ; the Terebratulce are 

 also attached by means of their tendons ; while 

 the Linguloe perforate the mud in more shallow 

 situations. 



