SHARKS AND RAYS. 513 



The anal fins are situated very far back on the elongated body, so as 

 to be separated by only a short distance from the caudal. It is 

 only in the pectoral fins that the lobate structure is visible externally, 

 each of these consisting of a central scaled portion, surrounded on the 

 free margins by a fringe of rays. The bichir, which grows to a length of 

 four feet, is characterised by the moderate elongation of the body, and the 

 presence of well-developed pelvic fins. These last are wanting in the much 

 smaller reed-fish, which takes its name from the excessive attenuation of 

 the body. Information is much wanting as to the life-history of these fishes, 

 although it is known that they lay small eggs. If we may judge from the 

 analogy of the Australian lung-fish, which has a similar type of fins, it is, 

 however, probable that they are comparatively slow and sluggish in their 

 movements, and live on, if not in, the mud at the river bottoms. In the late 

 larval state, Polypterus, at any rate, develops a large external gill on each 

 side, projecting backwards from the gill-opening. In addition to the fringing 

 dorsal fin, the tubular nasal openings may be noticed as peculiar characters 

 of the existing forms. It may be added that some authorities like Dr. 

 Steindachner recognise more than a single species of bichir, one of them 

 being named Polypterus lapradei. 



SUB-CLASS IV. ELASMOBRANCHII. 

 SHARKS AND RAYS. 



THE dreaded sharks are the types of a fairly large sub-class of 6shes, which 

 also includes the smaller but very similar dog-fishes, as well as the more 

 aberrant saw-fishes, and the skates and rays. The latter are, indeed, so 

 unlike sharks in general appearance, that it is probable their affinities to the 

 latter are not generally recognised. Nevertheless, the two groups agree in 

 all essential structural features, and are evidently very nearly allied. Per- 

 haps the most characteristic feature of the sub-class, and the one from which 

 it derives its title of Elasmobranchii, is the conformation of the gills and their 

 apertures. Everybody knows the five vertical slits on each side of the neck 

 of a dog-fish. These are the apertures leading into the gill-chambers; and 

 in the latter the gills themselves are attached by their margins to the skin. 

 It will not fail to be noticed that in this type of structure there is nothing 

 comparable to the gill-cover or operculum of the bony fishes and their allies. 

 Still there is a very important point of connection between the Elasmo- 

 branchii and the Teleostomi ; this being a feature whereby these two sub- 

 classes are broadly distinguished from the Dipnoi and Holocephali. To 

 explain this peculiarity in detail requires the use of a number of technical 

 terms; and it must suffice to state that it is connected with the manner in 

 which the suspending apparatus of the lower jaw is attached to the skull ; 

 the attachment in this case being movable, whereas in the other two sub- 

 classes it is immovably fixed. It has been said above that the number of 

 gill-slits in a dog-fish is five, and this number is very characteristic of the 

 order generally. Still, however, there are a few exceptions, and the number 

 of slits may be increased to six or even seven on each side of the neck. 

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