THE BRITISH NATURALIST. 



63 



HOW DO ANIMALS BREATHE? 



BY A. J. JOLLEY. 



(Continued from page 13.) 



The place of the so-called tracheo-branchiae is, in the larvae ot dragon 

 flies, taken by folds of the walls of the rectum. These folds contain 

 numerous fine tracheae, and these obtain oxygen from water which is 

 constantly pumped into and out of the rectum. 



Many of the aquatic Arthropods, for instance crabs and lobsters, have 

 true gills, which are externally projecting processes of the body or limbs, 

 but in this group, as in the others already referred to, many of the lower 

 members possess no special respiratory organs. 



Perhaps the most remarkable development ot the Arthropod breathing 

 apparatus is found in some Arachnidans, as the group to which the 

 scorpions, spiders, and mites belong is called. 



Scorpions breathe by pulmonary sacs which are also possessed by 

 spiders. The scorpions usually have four pairs of these sacs ; each has 

 one external opening, and internally is broken up into a number of folds 

 in which the blood circulates and takes up oxygen. The folds present 

 an appearance not unlike the partitions of a purse or pocket book, with 

 the difference that a purse usually contains only two or three folds, whilst 

 the arachnidan's sac contains perhaps twenty. 



Spiders have one or two pairs of these sacs in addition to a tracheal 

 system, somewhat like that of the insects. Water spiders, like water 

 insects, do not possess gills ; they take down minute air-bubbles amongst 

 the hair-like growth which clothes their bodies, and some take down a 

 large bubble held between the spinnerets. 



In the curious group Echinoderms, to which the star-fishes belong, there 

 are no special respiratory organs, unless certain small plume-like develop- 

 ments are to be regarded as gills. Like many other animals which are 

 destitute of special respiratory organs, the oxygen necessary to the life of 

 the animal enters the system through the alimentary canal ; it is evident 

 that a considerable quantity of water enters with the food and the 

 oxygen held in solution enters into the system through the walls of the 

 alimentary canal. 



We have now to examine the great division of the animal kingdom to 

 which we ourselves belong, and will first notice a distant cousin, the 

 Balanoglossus. This remarkable little worm possesses a dorsal rod 

 closely resembling the notochord of young lampreys and sharks ; its 

 central nervous system is dorsal, and its gill apparatus is very similar to 

 that of the lancelet. In its larval form it closely resembles the young of 

 some of the Echinoderms, and we have thus very suggestive hints of the 

 relationships of forms apparently widely separated, and from the life 



