FISH. 



323 



lucli as the Climbing Perch, Anabas). The gills are covered by a 

 )ony plate, the opercidmn (fig. 165, o). The limbs when present 

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

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

 iricle. The blood runs back from the body into the auricle {a), 

 md from thence through the ventricle 

 .t 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. 

 rhe blood is only pumped to the gills ; 

 thus the heart is a purely respiratory 

 Dne. A skeleton is always present, 

 either bony or cartilaginous. In the 

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

 IS cartilaginous ; in the more highly 

 ieveloped fish — Teleostei — it is bony, 

 rhe sexes are always distinct, ova or 

 spawn being deposited by the females, 

 rhe young fish or " fry " are like the 

 parent ; hut a few, such as the Lam- 

 preys, have a kind of metamorphic 

 ievelopment. The embryo fish has 

 10 amnion, and the allantois, which is 

 represented by the urinary bladder, is 

 ilways rudimentary. Mont fishes are 

 iovered by scales, which are formed 

 3y the dermal or under layer of the 

 ikin. The vertebrae are always bi- 

 ioncave or amphicoelous, the concav- 



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

 nous fishes, such as the sharks, we can see the vertebra being 

 Irawn into the skull to take part in its formation. As, with 

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

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

 nay thus summarily dismiss them. 



Fig. 166. — Diagram of the Cir- 

 culation IN Fishes. 



a. Auricle ; v, ventricle ; 7)i, 

 bulbus arteriosus ; n, branchial 

 artery ; 6, vessels in gills ; c, 

 aorta. (Nicholson.) 



