110 ZOOLOGICAL EXERCISES. 
dosteus) a ganoid, in which they are opisthoce- 
lous. 
Fishes are divided into the following sub-classes: 
Pharyngobranchii. No brain, nor auditory 
organ ; heart a simple tube; respiration car- 
ried on by the perforated walls of the 
pharynx. 
Marsipobranchii. Skull cartilaginous ; noto- 
chord persistent ; heart of two chambers, but 
no bulbus arteriosus; gills sac like pouches ; 
nasal sac single. 
Elasmobranchii. Skeleton cartilaginous; nasal 
sac double; heart with ventricle, auricle, 
and contractile bulbus arteriosus, with several 
rows of valves; gills attached to the body 
wall. 
There are two orders : 
Holocephala. Only one external gill opening ; 
jaws coalescent with the skull. 
Plagiostomata. From five to seven gill open- 
ings ; jaws distinct from the skull. 
Ganoidei. Skeleton partially ossified; heart 
with ventricle, auricle, and contractile bulbus 
arteriosus, with several rows of valves; gills 
free. 
Teleostei. Skeleton ossified ; heart with ven- 
tricle, auricle, and non-contractile bulbus ar- 
teriosus, which has only one row of valves; 
gills free. 
There are five orders: 
Plectognatht. Skin with bony scutes or 
