THE FROG AND VERTEBRATES IN GENERAL 155 



Class II. Elasmobranchii. This class includes a number of 

 species, some of which occur in great abundance in most marine 

 waters. The Sharks, Dogfish (a species of Shark), and Rays, or 

 Skates, belong to the Elasmobranchs. These animals show con- 

 siderable advance over the Cyclostomes. Thus (a) the notochord 

 is segmented, only partially persistent, and cartilaginous vertebrae 

 have arisen ; (6) a well-developed lower jaw is present and pos- 

 sesses modified scales which serve as teeth ; (c) two pairs of 

 lateral fins are found. (W. fs. 69, 70, 120.) 



There are two subclasses of Elasmobranchs recognized, namely, 

 the Selachii and the Holocephalii. The last named subclass is 

 small in numbers and relatively unimportant from our standpoint, 

 but considerable interest attaches to the Selachian group. It is 

 very clearly divided into two orders, to the first of which belong 

 all the many species of Sharks, and to the second of which belong 

 the Skates and Rays. The Sharks are slender types, such as the 

 Dogfish, with gill slits on the side, and with the body almost 

 circular in outline as seen in transverse section, while the Rays 

 and related forms are decidedly flattened dorso-ventrally and with 

 the gill slits on the under, or ventral, surface. Thus they are 

 perfectly adapted for living conditions on the sea-bottom. The 

 common Dogfish, Squalus acanthias, is used a great deal for labo- 

 ratory study in comparative anatomy as an important example 

 of the lower Vertebrates. Among the Rays, all of which exhibit 

 a number of typical features, the Torpedo is particularly note- 

 worthy because of the amazing modification of certain muscles 

 lying in the head region which permit them to accumulate charges 

 of electrical energy sufficient to paralyze large animals. 



Class III. Pisces. This is by far the largest and most impor- 

 tant group of the 'fish-animals,' including, as it does, some 15,000 

 species of the so-called ' bony fishes, ' among which are to be found 

 practically all the more common fresh and salt water types, such, 

 for example, as the Perch, Cod, Trout, and Salmon. Again con- 

 siderable advances in organization over both the Cyclostomes and 

 Elasmobranchs are to be noted. In fact the Pisces are often 

 referred to as the 'true fishes.' Of outstanding importance is the 

 fact that, for the first time, bone is developed in the endoskeleton. 

 In many species of this class the skeleton is almost entirely ossified, 

 while in others the original cartilaginous skeleton is only replaced 

 in part. As a rule the notochord is entirely replaced by the seg- 



