232 VERTEBRATE ANIMALS. 
“rays.” These rays are sometimes simple undivided rays or 
spines, when they are called “spinous rays” (123, @); but in 
other cases they are both divided by transverse joints, and split 
up into numerous longitudinal branches towards their extremi- 
ties, when they are spoken of as “soft rays” (123, @’). The 
soft rays occur in many fishes in different fins, but they are 
invariably present in the caudal fin or tail. 
As regards the digestive system in fishes, the mouth is usually 
furnished with a complicated system of teeth, developed not 
only upon the jaws, but upon any or every bone which enters 
into the composition of the oral cavity. The gullet opens into 
a stomach, usually of large size, and its hinder aperture (the 
Pylorus) is usually furnished with a valve. Immediately be- 
hind the pyloric opening of the stomach there are usually a 
variable number of blind tubes (called the “ pyloric czca”) 
which open into the intestine, and which are believed to repre- 
sent the pancreas. In some fishes, however, there is a well- 
developed pancreas, and in others even these tubes are wanting. 
The intestinal canal is a longer or shorter, more or less con- 
voluted tube, and its absorbing surface is sometimes largely 
increased by a spiral folding of the mucous membrane, which 
winds like a screw in close turns from the pylorus to the anus. 
The liver is usually of large size, and saturated with oil, but in 
the Lancelet it is doubtfully represented by a hollow sac-like 
organ. The kidneys in fishes are of great comparative size, 
forming two elongated organs, situated beneath the spine, and 
extending along the whole length of the abdomen. 
Respiration in all fishes is aquatic, and is effected by means 
of gills or dvanchia, in all except the Lancelet, in which respira- 
tion is effected by branchial filaments placed round the pharynx, 
and also by a greatly developed pharynx perforated by ciliated 
apertures (fig. 127). The arrangement and structure of the gills 
in fishes vary a good deal in different orders, and the leading 
modifications will be noticed hereafter, In the meanwhile it 
will be sufficient to give a short description of the branchial 
apparatus in one of the bony fishes. In such a fish the gills 
consist of a single or double series of flat cartilaginous leaflets, 
covered by mucous membrane, richly supplied with blood, and 
arranged on bony or cartilaginous arches which are connected 
with the tongue-bone (Ayo/d bone) below and with the under 
surface of the head above. The branchial arches and branchize 
