74 Connecting Links 



They must be distinguished from structures which 

 have dwindled, yet have been transformed into some- 

 thing else that is useful. As we have mentioned, the 

 sting of a worker-bee or wasp is a small version of an 

 ovipositor, but it cannot be called vestigial, since it 

 forms a very effective weapon. The few hairs some- 

 times seen on a whale's upper lip are in a sense 

 "relics" of the hairy covering which the ancestors 

 of whales probably possessed, but individually they 

 are anything but vestigial. They are very richly 

 innervated and are doubtless important as tactile 

 structures, like the vibrissa? on the cat's face. 



At many levels in the Animal Kingdom there are 

 living annectent types or connecting links which bind 

 one group to another. This may be illustrated by the 

 Mudfishes or Dipnoi, which are intermediate between 

 Amphibians and ordinary fishes. Many new acquisi- 

 tions were made when Amphibians emerged from a 

 piscine ancestry, perhaps from a stock related to 

 the f ringed-finned fishes (Crossopterygii), and some 

 of these acquisitions are partly anticipated by the 

 Mudfishes. Thus the ordinary swim-bladder of fishes 

 has become a genuine lung in the Mudfishes, which 

 thus point onwards to the lunged Amphibians. An 

 ordinary fish has unicellular skin glands; frogs and 

 other Amphibians have multicellular skin glands ; but 

 these are also found in the Mudfishes. The heart of a 

 fish is two-chambered, with a receiving chamber or 

 auricle and a driving chamber or ventricle ; the heart 

 of an Amphibian is three-chambered, with two auri- 

 cles and a ventricle; the heart of a Mudfish is in- 



