16 



epithelial lining of the gut, and the associated organs (e.g., 

 lungs, liver, etc.), and also the foundation of the endo- 

 skeleton, the notochord. 



3. The Mesoderm forms the dermis, the reproductive 

 organs and ducts (which arise from its ccelomic wall), the 

 principal parts of the muscular system, and the coats of 

 the blood-vessels. 



Connective tissue, cartilage, and bone are derived from 

 the mesenchyme (which is formed from immigrant cells 

 budded off from the epiblast or the hypoblast of the gas- 

 trula), i.e., from mesoderm in which the cells are at first 

 discrete and amoeboid. 



What are (a) Homologous, (b) Analagous Organs ? Give 

 Examples. 



Organs in the same or in different animals which are 

 structurally similar and which are of similar origin are 

 homologous. Such are the various appendages of a cray- 

 fish. The pectoral fin of a fish, the fore-limb of a frog, 

 the wing of a bird, the flipper of a whale, and the arm of 

 a man are all homologous. 



Analagous organs have the same function but not the 

 same structure, e.g., wing of insect and wing of bird. 



The wing of a bird and the wing of a bat are both homo- 

 logous and analogous. 



Explain Convergence, or Homoplasy. 



When two or more animals, not closely related, have a 

 marked outward resemblance due to adaptation to en- 

 vironment, or to habit, they are said to exhibit convergence, 

 e.g., a burrowing amphibian (Ichthyophis), a burrowing 

 lizard (Slowworm), and a burrowing snake ( Typhlops). 



Distinguish between a Unicellular and a Multicellular 



Animal. 



A unicellular animal is one in which the body consists 

 of a single cell or unit of living matter (protoplasm) which 

 performs all the functions of life, namely movement, 



