258 ANIMAL PHYSIOLOGY. [LESSON ^8. 



1169. Each arm extends outwards, advances forwards, curves 

 slightly inwards, and bends abruptly back upon itself, the two parts 

 of the bend being connected together ; then the stem again curves 

 forward, and becomes united to the corresponding bend of the oppo- 

 site arm, the conjoined extremities describing spiral convolutions 

 turned towards the dorsal valve ; the bent portions of the fringed 

 arms are supported by slender and elastic calcareous processes. In 

 all the Brachiopods, the stem which supports the brachial fringe is 

 hollow. 



1170. In the Terebratulcs, and Orliculce, the spiral terminations 

 of the arms have their central canal surrounded by a double oblique 

 series of muscular fibres ; the canal is filled with fluid, and, by the 

 contraction of the muscular fibres, the extremities are extended by 

 the pressure of the contained fluid which is'injected into them. 



1171. The structure of the alimentary canal, and its accessory 

 appendages, have been described in their proper place (p. 159). 



1172. So, too, the organs of circulation, consisting, as some au- 

 thors (Owen) have supposed, in a heart placed on either side of the 

 alimentary canal, from whence the blood is distributed to all parts 

 of the body, especially to the vascular mantle. The probability that 

 these views are erroneous has been already stated (p. 158). 



1173. In the nervous system of Terebratula, we find a nervous 

 collar, with only one ganglionic enlargement, surrounding the oeso- 

 phagus the ganglion being placed above the latter (Fig. 377, a). 

 Nervous filaments are distributed to the ciliated arms (d), the adduc- 

 tor muscles, and to the vascular lobes of the mantle (5, b). 



1174. Two pairs of short, powerful muscles arise from each valve, 

 some of which are lost in the opposite valve, and others lost in the pedicle. 



1 175. The pedicle is surrounded by an elastic yellow horny layer 

 and a tubular prolongation of the mantle. 



1176. The true history of these interesting animals has been given 

 in the following beautiful and graphic language, by Prof. Owen : 

 " Both the respiration and nutrition of such animals, which exist be- 

 neath such an amazing pressure as a column of ninety fathoms (540 

 feet !) of sea-water, are subjects suggestive of interesting reflections, 

 and lead one to contemplate with less surprise the great strength and 

 complexity of some of the minutest parts of the frame of these di- 

 minutive creatures. In the unbroken stillness which must pervade 

 those abysses, their existence must depend upon their power of excit- 

 ing a perpetual current around them, in order to dissipate the water 

 already laden with their effete particles, and to bring. within the reach 



