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, é.g. the sturgeons, are often called ‘* Ganoids.”” 
Primitive Vertebrates.—Under this title we include— 
(1) the Roundmouths or Cyclostomata; (2) the lancelets 
or Cephalochorda; (3) the Tunicates, some of which are 
See Ses 
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Fic. 7.—A lancelet, Amphioxus.—After Haeckel. 
called sea-squirts; and (4), with much hesitation, several 
strange forms, especially Balanoglossus, which exhibit 
structures suggestive of affihity with Vertebrates. 
The Cyclostomata, represented by the lamprey (Pe/vo- 
myzon) and the hag (AZyxine), and some other forms, 
probably including an interesting fossil known as FPalgo- 
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 limbs, 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. 
