FISH. 



323 



such as tlie Climbing Perch, Anabas). The gills are covered by a 

 bony plate, the operculum (fig. 165, o). The limbs when present 

 are represented by fins ; and the heart, except in one group, the 

 Dipnoi, consists of two chambers only, a single auricle and a ven- 

 tricle. The blood runs back from the body into the auricle (a), 

 and from thence through the ventricle 

 it is sent to the gills (6) to be purified. 

 From the gills it runs on as arterial 

 blood to the various parts of the body. 

 The blood is only pumped to the gUls ; 

 thus the heart is a purely respiratory 

 one. A skeleton is always present, 

 either bony or cartilaginous. In the 

 lowest fishes — Sharks, Eays, &c. — it 

 is cartilaginous; in the more highly 

 developed fish — Teleosiei — it is bony. 

 The sexes are always distinct, ova or 

 spawn being deposited by the females. 

 The young fish or " fry " are Uke the 

 parent ; but a few, such as the Lam- 

 preys, have a kind of metamorphic 

 development. The embryo fish has 

 no amnion, and the allantois, which is 

 represented by the urinary bladder, is 

 always rudimentary. Most fishes are 

 covered by scales, which are formed 

 by the dermal or under layer of the bu'ibns"^riosSB T™, toan'chtai 

 skin. The vertebrae are always bi- fortZ^icho'^V ^"'= "' 

 concave or amphicoelous, the concav- 

 ities being filled in with notochordal matter. In the cartilag- 

 inous fishes, such as the sharks, we can see the vertebrae being 

 drawn into the skuU to take part in its formation. As, with 

 one or two exceptions, fishes are aquatic creatures, and as they 

 are of no importance agriculturally (unless it be as manure), we 

 may thus summarily dismiss them. 



Fig. 166. — ^Diaobam of the Cir- 

 cdlation ik 



