512 



BEACHIOPODA. 



Fig. 258. 



that, notwithstanding their calcareous nature, they possess a slight 

 degree of elasticity, and yield a little to pressure. The interspace 

 between the two folds of the calcareous loop is filled up by a strong but 

 extensile membrane, which binds them together, and forms a protecting 

 wall to the viscera ; the space between the bifurcated processes in 

 T. Chilensis is also similarly occupied by a strong aponeurosis. In this 

 species the muscular stem of each arm is attached to the outer sides of 

 the loop and the intervening membrane. They commence at the pointed 

 processes at the origin of the loop, advance along the lower portion, 

 turn round upon the upper one, are 

 continued along it till they reach 

 the transverse connecting bar, where 

 they again advance forwards, and 

 terminate by making a half-spiral 

 twist in front of the mouth. 



(1346.) The most obvious function 

 attributable to the tentacular organs 

 of the animals composing this class is 

 connected with the procurement of 

 food; for, being utterly deprived of 

 prehensile instruments, without some 

 adequate contrivance these helpless 

 creatures, imprisoned in their testa- 

 ceous covering, and fixed immoveably 

 in one locality, would be utterly un- 

 able to obtain the nourishment neces- 

 sary for their support. The pro- 

 vision for this purpose is found in the arms, whether they be extensible 

 or attached to calcareous loops ; for these organs, being covered by cilia, 

 produce powerful currents in the surrounding medium, which, being 

 directed towards the mouth as to a focus, hurry into the oral aperture 

 whatever nutritive particles may chance to be in the vicinity. The 

 mouth itself is a simple orifice with prominent fleshy lips (fig. 256, 6), 

 but unprovided with any dental apparatus. The alimentary canal in 

 Lingula is a long and convoluted tube, but without a perceptible sto- 

 machal dilatation ; in Terebratala, however, there is a large oval sto- 

 mach (fig. 258, A, d), into which numerous ducts derived from the hepatic 

 follicles open by large orifices. The structure of the liver in these 

 animals is displayed by Professor Owen in the memoir from which the 

 annexed figures are taken ; and the simplicity of its organization aifords 

 an interesting lesson to the physiologist. The hepatic organ (fig. 258, A, c) 

 consists essentially of numerous secerning caeca (fig. 258, B), as yet easily 

 separable from each other ; over which the visceral blood-vessels ramify, 

 and bring to the secreting sacculi the circulating fluid from which the 

 bile is elaborated. 



Digestive apparatus of Terebratula. 

 A: a, b, d y e, f, the alimentary canal; 

 c, the hepatic caeca. B : hepatic follicles, 

 magnified. 



