6 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



arches. There is an exoskeleton of scales, and the skin 

 also bears numerous glandular cells and sensory structures. 

 In many ways Fishes are allied to Amphibians, especially 

 if we include among Fishes three peculiar forms, known as 

 Dipnoi, which show hints of a three-chambered heart, and 

 have a lung as well as gills. Other Fishes have a two- 

 chambered heart, containing only impure blood, which is 

 driven to the gills, whence, purified, it passes directly to the 

 body. 



Apart from the divergent Dipnoi, there are three great orders of 

 Fishes — the cartilaginous Elasmobranchs, such as shark and skate ; the 

 Ganoids, such as sturgeon and bony pike ; and the Teleosteans or bony 

 fishes, such as cod, herring, salmon, eel, and sole. 



Primitive Vertebrates. — Under this title we include — (i) 

 the class of Roundmouths or Cyclostomata ; (2) the class 

 represented by the lancelets ; (3) the class of Tunicates, 



Fig. T.—Amphioxus. — After Haeckel. 



some of which are called sea-squirts ; and (4), with much 

 hesitation, several strange forms, especially Balanoglossus, 

 which exhibit structures suggestive of affinity with Verte- 

 brates. 



The Cyclostomata, represented by the lamprey (Petro- 

 myzon) and the hag (Myxine), and some other forms, 

 probably including an interesting fossil known as Palceo- 

 spondylus, are sometimes ranked with fishes under the title 

 Marsipobranchii. But they have no definitely developed 

 jaws, no paired fins, no scales, and are in other ways more 

 primitive. 



The lancelet (Amphioxus), for which the class Cephalo- 

 chorda has been erected, is even simpler in its general 

 structure. Thus there is an absence of limbs, skull, jaws 

 well-defined brain, heart, and some other structures The 

 vertebral column is represented by an unsegmented (or un- 

 vertebrated) rod, called the notochord, which in higher 



