434 ANIMAL RESPIRATION 



and their breathing organs are constructed on the type just 

 described. The movements of breathing can easily be watched 

 in a living specimen of the common Black Slug (Arion ater\ 

 which is one of the species in which the shell has disappeared 

 altogether. The lung is here rather small, and its boundary is 

 marked by a different texture of skin. As breathing goes on the 

 pulmonary aperture may be noticed alternately 'enlarging and 

 narrowing. 



Among the commonest air-breathing Snails living in fresh 

 water are different species of Pond-Snail (Limn&us) and Trumpet- 

 Snail (Planorbis). These and similar forms are obliged to come 

 to the surface from time to time for the purpose of breathing. 



AIR-BREATHING ARTHROPODS (ARTHROPODA) 



The typical air-breathing classes of the Arthropods are: 

 (i) the one which contains only the widely-distributed and simply 

 organized form Peripatus (Prototracheata), (2) Centipedes and 

 Millipedes (Myriapoda), (3) Scorpions, Spiders, Mites, &c. 

 (Arachnida), and (4) Insects (Insecta). There are also certain 

 Crustacea which have taken to a life on land, and have therefore 

 acquired the power of breathing ordinary air. 



PERIPATUS (PROTOTRACHEATA) 



This creature possesses in their simplest form the air-tubes 

 (trachea) which are the most characteristic respiratory organs 

 of air-breathing Arthropods. Indeed, on this account, these are 

 often collectively termed Tracheata. 



Scattered over the body of Peripatus are a number of small 

 holes (stigmata), some of which are arranged on either side in 

 two rows, one above and the other between the stump-like legs. 

 Each of these holes opens into a small cavity from which arise 

 a number of delicate air-tubes that penetrate the body to some 

 extent, the end gained being to supply oxygen to the different 

 organs, and take away their carbonic acid gas together with 

 some water vapour. By means of alternate contraction and 

 relaxation of the muscular wall of the body the air in these 

 tubes is from time to time renewed. The principle of this 

 arrangement is quite different from that exemplified by the 



