IX DIGESTION RESPIRATION EGGS 331 



stream of this water, which is not, as is generally supposed, the 

 urine from the bladder. 



The mode of respiration is interesting. The lungs are very 

 complicated, highly - developed, spongy structures. They are 

 attached by their whole dorsal surface to the inner lining of the 

 shell. As they cannot expand through their own initiative, 

 and since the shell has made costal and abdominal expansion 

 impossible, the tortoise has to resort to other means of producing 

 the necessary vacuum. This is done partly by the neck and the 

 limbs, which act like pistons in being drawn in and out ; partly 

 by the greatly developed hyoidean apparatus, by which, when 

 the neck is stretched out, the throat is alternately inflated and 

 emptied, the air being swallowed, or pumped into the lungs. 

 Additional respiration, besides that of the anal sacs mentioned 

 above, is effected in various aquatic tortoises by slightly vas- 

 cularised recesses of the pharyngeal region. Most Chelonians 

 can exist for a very long time without breathing; sulky individuals 

 remain for hours or days under water. Cistudo can shut itself 

 up for an equally long time. Nevertheless this and other land- 

 tortoises easily get drowned. 



All Chelonians lay white eggs, round or oval, according to 

 their kind, but the shape of the eggs of one set sometimes varies 

 within the greatest limits. The shell varies from a parchment- 

 like, flexible, scarcely calcareous cover to a hard, well-polished 

 case. As a rule the eggs, imbedded in the ground, are hatched 

 after a few months, but in some of the northern kinds, e.g. Emys 

 orbicularis, the hatching is deferred until the next spring, the 

 embryo's development being arrested during the winter. How 

 such eggs, buried a few inches only below the surface, withstand 

 the often very severe North German and Eussian winter is a 

 mystery. Whilst the plastron is generally flat, it is more or less 

 concave in the males of many species, notably in Testudo, 

 Cistudo, and Emys. 



The general conclusions which can be drawn from the present 

 geographical distribution of the Chelonia are as few and unsatis- 

 factory as those applying to the Crocodilia, since all the main 

 groups of Chelonians, and many more extinct families, occurred 

 together in bygone ages in the same countries, for instance in 

 Europe. The marine forms are naturally cosmopolitan, but the 

 Testudinidae are likewise cosmopolitan, except in the Australian 



