970 LIFE : OUTLINES OF GENERAL BIOLOGY 



Amphibia, which seems to have very Httle functional importance 

 and has no direct connection with the kidneys. In Amphibians, 

 then, the future allantois is an adult, not an embryonic organ; in 

 higher Vertebrates the reverse is on the whole true, though the 

 stalk of the allantois seems to become the permanent urinary bladder. 

 Another great change is that the allantois in higher Vertebrates 

 (Amniota) is a very vascular organ, which is far from being the 

 case in Amphibians. In this case, then, there is {a) a change in 

 function and in minute structure; (b) a shunting of the major 

 functional importance from the adult in one group to the embryonic 

 life and use in higher ones ; and (c) the preservation of a part of this 

 embryonic structure which becomes utilised in Mammals and most 

 Reptiles to form a new adult organ, the urinary bladder. 



(4) Another striking illustration of function-change is afforded by 

 the three linked ear-ossicles of mammals, which convey vibrations 



Fig. 165. 



A Section through the Thorax Region of the Robber Crab (Birgus latro). 

 After Semper, i, overlap of the cephalothorax shield; 2, respiratory- 

 tufts; 3, two small gills; 4, base of a leg; 5, the ventral nerve cord; 6, the 

 food-canal in section ; 7, the pericardium around the heart. 



from the drum to the internal auditory organ. Their names, from 

 without inwards, are malleus or "hammer", incus or "anvil", and 

 stapes or "stirrup", the first abutting on the tympanum, and the 

 third fitting into the fenestra ovalis of the inner ear. In Birds, 

 Reptiles, and Amphibians, their place is taken by a single rod, the 

 columella; in Fishes there is no drum, and therefore there are no 

 such ear-ossicles, the calcareous masses, called otoliths, being much 

 simpler in nature and origin. But it is possible, from the development 

 of the mammalian ossicles, to argue back to their homologies in the 

 other Vertebrates, including fishes; and the generally accepted 

 conclusion is that the malleus corresponds to the articular bone of 

 the lower jaw in the other Vertebrates; the incus to the quadrate, 

 the jaw-hinge bone in Birds, Reptiles, and Bony Fishes; and the 

 stapes probably to the upper end of the hyoid arch, represented, for 

 instance, by the hyo-mandibular of the haddock. Even if subsequent 

 investigation should necessitate some modification of these homolo- 

 gies, it will not affect the general argument that the delicate chain 



