the most primitive metazoa 69 



Origin from the Coelenterata 



This view, that the coelenterates and the Porifera have close 

 ancestral affinities is quite an old one. It is based on the fact that 

 the coelenterates and the sponges often have a solid planula-type 

 larva during their embryology. Lankester (1890) strongly sup- 

 ported this view when he emphasised the differences between the 

 planula larva and the Gastrula. He thought that there was no 

 indication that the sponges ever had a gastrula stage but that 

 instead the resemblance was in the solid blastula. 



In Leucosolenia the fertilised egg divides to form a sixteen- 

 celled hollow blastula. The majority of the cells are flagellated but 

 a few at one end are non-flagellated. These non-flagellated cells 

 together with a few of the flagellated cells migrate into the interior 

 of the blastula and fill the central cavity. The result is a solid 

 blastula. This settles on the ground, flattens and the inner cells 

 then migrate out on top of the flagellated cells. They then become 

 the pinacocytes and amoebocytes, whilst the flattened flagellated 

 cells turn into the choanocytes. 



Though there are certain similarities between the development 

 of the sponges and the coelenterates, it is difficult to know how 

 much reliance can be placed on them. Thus Balfour (1880) was 

 quite clear that there was no relationship between the cell layers 

 of the sponges and those of the coelenterates since the inner layers 

 of the sponge embryos come to lie on the outside of the adult. 

 For the same reason Delage (1898) suggested that the sponges have 

 their endoderm on the outside and the ectoderm on the inside and 

 that the sponges should be called the " Enantiozoa " for this 

 reason. It is of interest that Saville Kent (1880) described the 

 way in which the outer cells of the sponge, the pinacocytes, could 

 take in food particles. 



It is possible to enumerate the similarities and differences 

 between the sponges and the coelenterates as follows. 



Similarities between sponges and coelenterates 



(1) They are both aquatic and free-living animal groups. 



(2) They have spicules in their skeleton, which are either 

 calcareous or horny. 



(3) They have flagella. 



