500 MANUAL OF ZOOLOGY. 



each of numerous vertical gelatinous columns, separated by membranous 

 septa, and richly furnished with nerves from the eighth pair ; the whole 

 arrangement 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 equiva- 

 lent 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 (Cephaloptera], and the 

 Beaked Rays (Rhinobatis) . 



In the Saw-fish (Pristis antiquorum} the body has not the typical flat- 

 tened 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. Though principally inhabiting the 

 sea, the Saw-fishes are not wholly marine, fresh-water forms occurring in 

 Nicaragua and in the Philippines. 



Before leaving the Elasmobranchii, a few words may be said as to their 

 position in the class of fishes. From the cartilaginous nature of the en- 

 doskeleton, and the similarity between the form of their gills and those 

 of the Lampreys and Myxinoids, 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 consideration the sum 

 of all their characters, there can be little hesitation in placing the order 

 nearly at the summit of the entire class. The nervous system, and espe- 

 cially the cerebral mass, is very much more highly developed proportion- 

 ately 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 eyelid (membrana nictitans), and the 

 nasal sacs having a very complex 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 branchiae, this being 

 a decided approximation to the Amphibia. 



ORDER VI. DIPNOI ( = Protopttri, Owen). This order is a 

 small one, and includes no other living forms than Mud-fishes 

 (Lepidosiren and Ceratodus} ; but it is nevertheless of great 

 importance as exhibiting a distinct transition between the fishes 

 and the Amphibia. So many, in fact, and so striking, are the 

 points -of resemblance between the two, that until recently the 

 Lcpidosireu (fig. 272) was always made to constitute the lowest 

 class of the Amphibia. The highest authorities, however, now 

 concur in placing it amongst the fishes, of which it constitutes, 

 with Ceratodus, the highest order. The order Dipnoi is defined 

 by the following characters : The body is fish - like in shape. 

 There is a skull with distinct cranial bones and a lower jaw, but 

 the notochord is persistent, and there are no vertebral centra, nor 

 an occipital condyle. The exoskeleton consists, in the living types, 

 of horny overlapping scales, having the "cycloid" character ; but 



