438 CHORDATA. 
They are world-wide in distribution, and are freshwater, 
pelagic, littoral, katantic and abysmal in habitat. 
They are divided into two unequal divisions — the 
Crossopterygit, mainly extinct but including olypterus of 
the Nile, and the Actinopterygii which embrace the 
remainder. These are in their turn divided into three 
sub-orders :— 
1. The Chondrostei are mainly extinct types, together 
with the sturgeon of sub-arctic regions and one or two 
species found in North America. Their skeleton is 
cartilaginous. 
2. The Aolostei include the bony pike (Lepzdosteus ) 
of North America and several extinct forms. The skeleton 
is osseous and there is a spiral valve in the intestine. 
3. The Zeleostei * constitute an immense number of well- 
known fishes. Their skeleton is osseous, they usually have 
horny (cycloid or ctenoid) scales, they have no conus 
arteriosus to the heart and no spiral valve in the intestine. 
Their principal groups are as follows :— 
1. Physostom? (air-bladder communicating with the cesophagus), 
most freshwater fishes and common marine forms, such as 
herring, sprat, eels. 
2. Anacanthint (air-bladder closed, the fins are soft), comprising 
cod, haddock and the flat-fish. (In the flat-fish the air- 
bladder is absent.) : 
3. Acanthopteri (fin-rays are spiny, air-bladder closed), including 
perch, mackerel, gurnard. 
4. Plectognathi and 5. Lophobranchit, two small groups with very 
specialised members. The Plectognathi usually have a 
hard bony exoskeleton and few powerful teeth with certain 
bones of the jaw fused. The Lophobranchii have tufted 
gills and may assume peculiar shape and habits; they 
include the pipe-fishes and the sea-horses. 
OrvDER Il.—Zlasmobranchit. 
This order includes the Sharks and Skates. Their tail 
is heterocercal, the scales are placoid. The gill-slits are 
* The archaic freshwater types like the sturgeon, bony-pike, and Polypterus 
all the extant Teeostomi, except Teleostez) used to be combined in an order called 
Ganoidei, but their genetic relationships to the Teleosted are now recognised. 
