DIPNOI, 399 



each side of the head, and consisting each of numerous vertical 

 gelatinous columns, separated by membranous septa, and richly- 

 furnished with nerves from the eighth pair ; the whole arrange- 

 ment presenting a singular resemblance to the cells of a voltaic 

 battery. There is no doubt, however, but that the force which 

 is expended in the production of the electricity is only nerve- 

 force. For every equivalent of electricity which is generated, 

 the fish loses an equivalent of nervous energy ; and for this 

 reason, the production of the electric force is strictly limited by 

 the amount of nerve-force possessed by the animal. 



Other well-known members- of the family are the Sting-rays 

 (Trygon), the Eagle-rays (Myliobatis), the Horned Rays {Cepha- 

 loptera), and the Beaked Rays {Rhinobatis). 



In the Saw-fish (Pristis antiquorum) the body has not the 

 typical flattened form of the Rays, and the snout is elongated 

 so as to form a long sword-like organ, the sides of which are 

 furnished with strong tooth-like spines. This constitutes a 

 powerful weapon, with which the Saw-fish attacks the largest 

 marine animals. 



Before leaving, the Elasmobranchii, a few words may be said 

 as to their position in the class of fishes. From the cartilag- 

 inous nature of the endoskeleton, and the similarity between 

 the form of their gills and those of the Lampreys and Myxin- 

 oids, the Elasmobranchii were long placed low down in the 

 scale of fishes, to which also the permanently heterocercal tail 

 conduced. When we come, however, to take into considera- 

 tion the sum of all their characters, there can be little hesita- 

 tion in placing the order nearly at the summit of the entire 

 class. The nervous system, and especially the cerebral mass, 

 is very much more highly developed proportionately than is 

 the case with any other division of the fishes. The organs of 

 sense are, comparatively speaking, of a very high grade of 

 organisation, the auditory organs being more than ordinarily 

 elaborate, the eyes being sometimes furnished with a third eye- 

 lid [membrajia niditans), and the nasal sacs having a very com- 

 plex structure. The structure of the heart agrees with that of 

 the Ganoids, and is a decided advance upon the heart of the 

 more typical bony fishes. Finally, the embryo, before its 

 exclusion from the egg, is furnished with external filamentous 

 branchise, this being a decided approximation to the Amphibia. 



Order VI. Dipnoi ( = Protopteri, Owen). — This order is 

 a very small one, and includes only the singular Mud-fishes 

 (Lepidosireii) ; but it is nevertheless of great importance as 

 exhibiting a distinct transition between the fishes and the Am- 

 phibia. So many, in fact, and so striking, are the points of 



