CHAP. V. RESPIRATION IN LAND CRABS. 37 



ordinary situation are very much reduced, and when 

 they are in the water their flagella never perform the 

 peculiar beating movements which may be observed in 

 other Crabs, and even in the larger Gelasimus ; more- 

 over, the organ of smell must probably be sought in 

 these air-breathing Crabs, as in the air-breathing Verte- 

 brata, at the entrance to the respiratory cavity. 



So much for the facts with regard to the aerial respi- 

 ration of the Crabs. It has already been indicated 

 why Darwin's theory requires that when any peculiar 

 arrangements exist for aerial respiration, these will be 

 differently constructed in different families. That ex- 

 perience is in perfect accordance with this requirement 

 is the more in favour of Darwin, because the schoolmen 

 far from being able to foresee or explain such profound 

 differences, must rather regard them as extremely sur- 

 prising. If, in the nearly allied families of the Ocy- 

 podidse and Grapsoidse, the closest agreement prevails 

 in all the essential conditions of their structure ; if the 

 same plan of structure is slavishly followed in every 

 thing else, in the organs of sense, in the articulation of 

 the limbs, in every trabecula and tuft of hairs in the 

 complicated framework of the stomach, and in all the 

 arrangements subserving aquatic respiration, even to 

 the hairs of the flagella employed in cleaning the 

 branchise, why have we suddenly this exception, this 

 complete difference, in connexion with aerial respira- 

 tion? 



The schoolmen will scarcely have an answer for this 

 question, except by placing themselves on the theo- 



