56 THE MOST PRIMITIVE METAZOA 



(4) They have a gut. (The Mesozoa have no gut.) 



(5) The gemmules with which some sponges carry out asexual 

 reproduction are quite complex in structure. 



(6) They have eggs and sperm. 



(7) The embryo gastrulates in a complex manner, by ingression, 

 epiboly and delamination. 



(8) They have a well-developed amphiblastula larva. 



(9) The blastopore is aboral, in most other metazoans it is oral. 



(10) An inversion of the layers occurs during embryology. 



(11) The sponges are the only animals to have the main body 

 aperture the exhalant one. 



Various excuses can be made for each and every one of the simple 

 or specialised characteristics. Thus, to deal with a few of them : 



(1) Though it is correct that the layers are loosely organised this 

 may be due to the fact that the sponges do not depend on the 

 hydraulic pressure of the gastro-vascular cavity to maintain their 

 shape. They have a well-developed skeletal system that takes care 

 of this. The animal remains intact even though there are many 

 series of canals running through the body, and the cells are not 

 firmly cemented together. 



(2) The layers do not correspond to the ectoderm, endoderm 

 and mesoderm of the higher animals and there is no evidence that 

 at one time they did correspond. There is not one piece of evidence 

 to show that the normal triploblastic condition evolved from that 

 shown by the sponges. 



(3) Many sponges such as Euplectella and Poterion do have a 

 definite shape. 



(4) Choanoflagellate-like cells are also found in the endoderm 

 of some coelenterates, annelids and molluscs. Electron microscope 

 studies show that the collar is a series of protoplasmic filaments 

 that project out of the cell in much the same way that the digestive 

 filaments project from an endodermal cell (Rasmont et al. 1958). 



(5) There are other animals that have no nervous system, e.g. 

 the Mesozoa. 



(6) The Mesozoa, Coelenterata, Ctenophora and Acoela have 

 no specialised excretory system. 



(7) The gut in the sponges though it shows little differentiation is 

 at least a gut. There is no gut in the Mesozoa, Acoela or Pogonophora. 



