Biology Today — December 2017

• Ornithorhynchus (Duck billed platypus) and Tachyglossus (Spiny ant-eater) are egg laying mammals. These act as
  connecting links between mammals in having hair, diaphragm, mammary glands, etc., and reptiles in having large coracoid,
  being oviparous, laying shelled and polylecithal eggs and having cloaca. So, the prototherians are commonly called unfinished
  mammals.
  Other well known connecting links are : Sphenodon, a primitive reptile with amphibian characters (e.g., amphicoelous vertebrae);
  Balanoglossus between chordates and non-chordates; Chimaera (rabbit fish) between cartilage and bony fishes; Peripatus
  (walking worm), between annelids and arthropods; Coelacanth between bony fishes and amphibians; Proterospongia between
  protozoa (amoeboid cells) and sponges (with collar cells), and Neopilina between annelids and molluscs.

(IV) Vestigial organs

• The vestigial or rudimentary organs are useless remnants of structures or organs that might have been large and functional
  in the ancestors.

(a) Vestigial organs in man are :

• Coccyx (Tail bone) is a small, triangular bone formed by fusion of 4 reduced caudal vertebrae and present at the end of the
  sacrum. It became vestigial due to bipedal gait and sitting posture of man.


• Nictitating membrane (Third eyelid or Plica semilunaris) is thin, transparent and freely movable fold that is moved to
  clean cornea in frog, pigeon, apes, etc. It is vestigial in man as the cornea is cleaned by regular blinking of eye lids.


• Caecum and Vermiform appendix are very long sized in the herbivores like rabbit, horse, etc., and have symbiotic bacteria
  that secrete cellulase enzyme to digest the cellulose part of the plant food. These organs are vestigial in man as cellulose of
  plant food is simplified during the process of cooking, so there is no need of symbiotic digestion of cellulose.

Vestigial hair and mammae

on the chest of man

Hair Roots

Wisdom

toothMolars

Premolars

Canine

Incisor

Wisdom teeth in human beings Vestigial auricular muscles of the

pinna in man

Superior auricular

muscle

Posterior

auricular

muscles

Anterior auricular

muscle

Tail bone in human being

Sacrum

Coccyx

(Tail bone)

Vestigial organs in humans

Nictitating membrane in the eyes of man

Nictitating membrane

Large intestine

Caecum

Vermiform

appendix

Vestigial caecum and vermiform appendix in man

(b) Vestigial organs in plants are :

(i) Leaves are reduced to scales in Cuscuta, Asparagus, Ruscus, etc.

(ii) Cutin covered stomata on stems of cacti plants.

(iii) Non-functional flagella on cycad sperms that are passively transported to the egg cells.