256 ANIMAL PHYSIOLOGY. [LESSON 78. 



special sense, and this portion of the brain may be considered as ru- 

 dimentary. So, too, the transverse lobes (c, c), which form, in all 

 probability, the elements of organs hereafter destined to become 

 Optic lobes. It is curious, however, to observe that whilst the por- 

 tion assumed to be the supra-cesophageal ganglion is uniformly very 

 dark in color, perhaps caused by a dense aggregation of ganglion cor- 

 puscles, the lateral lobes are conspicuous for the great number of these 

 corpuscles they contain, which may be distinctly seen in the figure. 



1162. From this remarkable condition of the nervous system, it 

 appears highly probable that the Salpa, as such, is simply in a tran- 

 sition state, and it may eventually happen that, when we have culti- 

 vated a more perfect acquaintance with it, it will be found to pro- 

 gress to another form, and a higher state of development. 



LESSOR LXXYIII. 



NEEVOUS SYSTEM IN THE BEACHIOPODA. 



1163. In the Brachiopods (brachion, an arm; pous, a foot), a 

 class of Mollusca which anciently existed to an immense extent dur- 

 ing the secondary period, and are found most abundantly fossilized, 

 but now restricted to three Genera, i. e. y Lingula, Orbicula, and 

 Terebratula, we have animals of great physiological interest. 



1164. In Terebratula Australis, only one ganglion is found, as 

 in the Tunicata, from whence the nerves are distributed to the vascu- 

 lar mantle, the strongly ciliated arms (from which the class obtains 

 its name), and to all parts of the body. It is very difficult to find 

 this ganglion, and for a long time it eluded detection; it is beauti- 

 fully displayed, however, in a preparation, a copy of which is repre- 

 sented at Fig. 377, a. 



1165. The Terebratula lives at a great depth in the ocean not 

 less than from sixty to ninety fathoms / for security, it attaches it- 

 self to submerged rocks, by means of a strong pedicle which projects 

 through a hole in a beak-shaped prolongation of one of the valves. 

 All the individuals of this class are bivalves. 



1166. The dorsal layer of the mantle (Fig. 376) presents a very 

 interesting spectacle. At the upper surface two pairs of very short, 

 powerful muscles (a, b) may be seen, for the purpose of closing the 

 valves. These muscles are somewhat triangular in shape, and ter- 

 minate each in a short, strong tendon, of great beauty, situate at 



