THE MAIN LINES OF ANIMAL EVOLUTION 



this does not result in the immediate formation of a typical gastrula, but 

 rather it consists of a ball of ectodermal cells filled by a solid core of ento- 

 dermal cells. This type of larva is called a planula. Only later does this 

 entodermal core hollow out and a mouth (blastopore) break through to 

 form a typical gastrula. While the type of gastrulation with which Haeckel 

 dealt is known, for example in the starfishes, it is not widespread in the 

 Animal Kingdom, and it appears to be a secondary modification rather 

 than a primitive character. 



It is plausible, then, that the ancestor of the Eumetazoa may have been 

 a blastula-like colonial flagellate, in which there occurred a differentiation 

 between somatic and reproductive cells, as in living Volvox, and then a 

 differentiation between digestive cells and locomotor cells, with the for- 

 mer type moving into the interior of the organism to form either a gastrula 

 or a planula. Yet it is probable that decisive evidence on this basic ques- 

 tion will never be obtained. That this primitive metazoan was not identical 

 with any living type is almost certain. 



The three most primitive living metazoan phyla are the Mesozoa, the 

 Coelenterata ( = Cnidaria ) , and the Ctenophora. These phyla are gener- 

 ally radially symmetrical (having one differentiated axis), or else biradi- 

 ally symmetrical ( having two differentiated axes ) . Their general grade of 

 organization is more advanced than that of the Porifera, for, while there 

 are no organ systems, there are two well-defined tissues, the ectoderm and 

 entoderm (or epidermis and gastrodermis ) . In most coelenterates and in 

 the ctenophores, there is between these layers a jelly-like mass, the me- 

 soglea, which also includes some cells. Thus it is not strictly true, as is 

 often stated, that these phyla have only two cellular layers. 



Phylum Mesozoa. The correct phylogenetic position of the Mesozoa 

 is a much vexed question. From a structural viewpoint, they are the sim- 

 plest of metazoans, consisting of an outer, generally ciliated, layer of cells 

 enclosing a core of internal, reproductive cells (Figure 46), They thus 

 resemble a planula, but the internal cells are not digestive cells. Van 

 Beneden named the group in 1877 with the intention of indicating his 

 judgment that the group was extremely primitive, intermediate between 

 the Protozoa and the rest of the Metazoa. On the other hand all Mesozoa 

 are parasitic, and their life cycles are somewhat similar to those of the 

 digenetic trematodes. Many zoologists treat the group as a degenerate off- 

 shoot from the flatworms. If it could be shown decisively that the char- 



^0 ^ 



FiGtmE 46. Conocijema deca, a Typical Mesozoan. (From MacConnaughey, /. Para- 

 sitol, V. 43, 1957. ) 



135 



