70 



THE BRITISH NATURALIST. 



[March 



very convex, whilst on the other hand anyone who has seen the skull of 

 the same animal must have been struck by its concavity. 



The reason of this difference in appearance is, that in the living 

 animal there is a spiracular chamber communicating - externally with the 

 blow-hole, and internally with the nostrils ; there is a complicated system 

 of valves and muscles by which the expiratory and inspiratory act is very 

 rapidly performed, so that the animal only needs to be at the surface a 

 very short time, and while under water can use air stored in its spiracular 

 chamber. 



It will be noticed that throughout this paper no attempt has been 

 made to enlarge upon the chemistry of respiration, whilst some anato- 

 mical questions have been ignored or only incidentally referred to because 

 their discussion would have involved an extensive use of technical 

 phraseology and would not have affected certain general conclusions 

 which, at the risk of undue repetition, may be briefly summarized. 



It will, for instance, be evident that a classification of animals based 

 solely on the respiratory apparatus would be in the highest degree mis- 

 leading, and that the comparative anatomy of the breathing organs is 

 only useful when employed in connection with an examination of the 

 general structure of the organisms. 



The comparative complexity of the breathing organs of an animal is 

 a safe index of its position in any of the great groups so long as the 

 comparison is restricted to members of that group, but no longer. 



For instance, it is fair to say that a lung-breathing mollusc is a higher 

 animal than a gill-breathing one ; but it is not fair to say that a lung- 

 breathing mollusc is higher than a gill-breathing vertebrate ; in other 

 words, a snail is higher than an oyster, but not than a herring. 



Bearing this general rule in mind, let us briefly glance again at the 

 groups into which animals are divided. All protists and zoophytes and 

 some of the lower members of other groups are devoid of special breath- 

 ing organs : they either take in oxygen from the water through the 

 general surface of the body or else derive it from the water which enters 

 the alimentary canal with their food. The lowest special respiratory 

 organs are the water-vascular vessels which are possessed by some 

 worms, and these form the starting point of a system whose highest 

 development is seen in insects, and whose intermediate stage is indicated 

 in the myriapods ; the fundamental structural arrangement is similar, 

 the highest stage being developed and modified for "air instead of water 

 breathing. 



In the same group as the insects we find two other methods of respira- 

 tion, viz., by gills as in the lobster, and by pulmonary sacs or lungs as in 

 the scorpions. Both these methods must be regarded as new develop- 

 ments, presenting no homology to the gills or lungs of any other groups 

 of animals. The gills of the higher molluscs are homologous to those 



