94 EVERS's COMPARATIVE ANATOMY. 



Gyclo-gangliata. — The greater number of this division breathe in 

 water, by means of gills, some however, breathe by lungs in the air. 

 The first mode of respiration is enjoyed by the tunicata, conchifera, 

 pteropoda, cephalopoda, and some of the gasteropoda, the remainder 

 of this class breathe atmospheric air, by means of a lung in the form 

 of a large cavity, placed beneath the mantle. A curious circumstance 

 connected with the conchifera is, that the eggs on escaping from the 

 ovary, are deposited between two layers of the branchial membrane, 

 where they increase in size and undergo incubation. 



CHAPTER XVI. 



ORGANS OP RESPIRATION IN THE VERTEBRATA. 

 PISCES. 



This entire class is covered with branchiae, by means of which 

 they abstract the free oxygen contained in the water. In the sharks 

 and rays, and in all the osseous fishes there are four gills on each 

 side, supported by as many branchial arches of cartilage or bone, 

 connected to the os hyoides. 



Each gill consists of a double series of lance-shaped lammellae 

 closed in by a movable cover, operculum. Generally there is but 

 a single opening for the passage of the water, but in some 

 instances, especially among the cartilaginous fishes, there are 

 several. In consequence of the swimming bladder of fish being 

 supposed by Cams, Blumenbach, and many others, to be subservient 

 to the purposes of respiration, it merits some notice in this place. 



This organ, which is sometimes divided by a septum, as in the 

 genus cyprinus, and sometimes absent, as in the pleuronectes, 

 Jophius, and mackerel, is placed in the abdomen, close to the spine, 

 and below the kidney ; consists of an internal vascular lining 

 membrane, a strong fibrous tunic, and a partial investment of peri- 

 toneum. In fresh-water fishes, it has been found to contain nitrogen, 

 and in salt water fishes, chiefly carbonic acid gas. From its 

 anterior part a canal, ductus pneumaticus, passes forwards and 

 opens into the oesophagus, except in the sturgeon where it 

 opens into the stomach. This duct is double in the cod, in the 

 carp it possesses valves so disposed as to admit of the egress, but 

 prevent the ingress of air; it is absent in the sciura, cobitis, burbot, 

 and others. When we consider that this organ is largest in such 

 fishes as swim with greatest velocity, and wanting in those where 

 large fins or a powerful tail compensate for its absence, we are dis- 

 posed to agree with those who regard it as an organ of progressive 

 motion. 



