6 GENERAL SURVEY OF THE ANIMAL KINGDOM 



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 the beginning of a three-chambered 

 heart, and have a lung as well as gills. Ordinary 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 two great orders of 

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

 and the Teleosteans or bony fishes, such as cod, herring, salmon, eel, 

 and sole. There are several smaller orders of great importance, some 

 of which, e.g. the sturgeons, are often called " Ganoids." 



Primitive Vertebrates. — Under this title we include — 

 (i) the Roundmouths or Cyclostomata ; (2) the lancelets 

 or Cephalochorda ; (3) the Tunicates, some of which are 



Fig. 7. — Lancelet, Amphioxus. — After Haeckel. 



called sea-squirts ; and (4), with much hesitation, several 

 strange forms, especially Balanoglossus, which exhibit 

 structures suggestive of affinity with Vertebrates. 



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 lancelets or Cephalochorda are even simpler in their 

 general structure (see Fig. 7). Thus there is an absence 

 of Umbs, skull, jaws, well-defined brain, heart, and some 

 other structures. The vertebral column is represented by 

 an unsegmented (or unvertebrated) rod, called the noto- 

 chord, which in higher animals (except Cyclostomes and 

 some fishes) is a transitory embryonic organ afterwards 

 replaced by the backbone. 



