70 THE MOST PRIMITIVE METAZOA 



(4) Amoebocytes are present in both groups. 



(5) The main body cavity is neither a haemocoel nor a coelom. 



(6) There are no excretory organs. 



(7) They occasionally have mesenchyme but never mesoderm. 



(8) They form buds or gemmules for asexual reproduction. 



(9) They have sexual reproduction with sperm and eggs. 



(10) They both form colonies. 



(11) They both have a high regenerative capacity. 



(12) They have a solid blastula larva (stereoblastula) before the 

 amphiblastula or planula larva develops. 



(13) The anterior end of the larva becomes attached to the 

 ground. 



(14) The sex cells are formed by interstitial cells or amoebocytes. 



Differences between sponges and coelenterates 



(1) The sponges have many entrances to the body cavity. 



(2) The main body opening of the sponges is the exhalant one. 



(3) The sponges have choanocytes (but see p. 56). 



(4) The outer layer of the sponge is never ciliated in the adult. 



(5) There are no nematocysts in sponges. 



(6) The sponges have no muscles or musculo-epithelial cells 

 (except porocytes). 



(7) The sponges have no nervous system or sense organs. 



(8) The sponges do not show polymorphism. 



(9) The sponge spermatozoa are sometimes carried to the egg by 

 amoebocytes. 



(10) The sponges never form a compact skeleton such as is seen 

 in certain coelenterates such as the corals. 



(11) The adult sponges are never pelagic. 



(12) There is no clear homology between the layers of the 

 sponges and the layers of the coelenterates. 



Though the above lists do not actually prove anything they 

 do indicate that the differences between the coelenterates and the 

 sponges are quite considerable and basic. It is thus doubtful if 

 there is any close relationship between these two groups. It is 

 also impossible to state whether the sponges arose earlier than the 

 coelenterates. Their organisation is less complex in some ways, 

 but this again does not necessarily mean that the sponges are 



