PHYLOGENY 



acters of the Mesozoa are primitive rather than degenerate, the group 

 would assume great phylogenetic importance, for it could then be reason- 

 ably argued that the group must be but little changed from the remote, 

 pre-Cambrian ancestor of the Metazoa. It would prove that the Metazoa 

 were derived from a planula- rather than from a gastrea-type ancestor, 

 and it would leave the Gastrea theory very badly damaged. But the avail- 

 able evidence does not furnish a basis for a final decision on the taxonomic 

 position of the Mesozoa. W. K. Brooks has said that "suspended judgment 

 is the greatest triumph of intellectual discipline, " and this appears to be 

 an appropriate place for that achievement. Hyman sets them off as an 

 independent branch of the Metazoa, the branch Mesozoa. 



Phylum Coelenterata. The Coelenterata have traditionally been 

 treated as the most primitive of the Eumetazoa. Haeckel regarded them 

 as the source of the flatworms, and hence of all higher phyla. The prin- 

 cipal reason for his viewpoint is the obvious resemblance of a hydroid 

 polyp to a gastrula. For the hydroid consists of two simple cell layers, with 

 no organ systems and with only a trace of noncellular mesoglea. The ana- 

 tomical structure of the polyp could be derived from that of the gastrula 

 simply by the elongation of the body and the drawing out of a circlet of 

 tentacles around the mouth. Food is taken in and waste residues expelled 

 by the mouth, which is simply the blastopore of the blastula. Food is still 

 digested by the protozoan method, that is, the cells of the gastrodermis 

 engulf food particles, and digestion is carried on intracellularly. But en- 

 zymes are also secreted into the gastrovascular cavity, and much digestion 

 occurs there. Muscle tails may be formed in connection with either the 

 epidermis or the gastrodermis. A nerve net is formed from epidermal ele- 

 ments. Thus there is a high degree of tissue differentiation, but no organ 

 systems. Outstanding specializations of the coelenterates include the de- 

 velopment of nematocysts, organelles for food procurement and defense; 

 and the alternation of a free-swimming, sexually reproducing medusa 

 generation with a sessile, asexually reproducing polyp generation. This is 

 unrelated to the alternation of generation of plants, for both generations 

 are diploid. Superficially so different, the polyp and the medusa are struc- 

 turally very similar, for one can derive the medusa from the polyp by 

 simply inverting the latter, greatly increasing the amount of mesoglea and 

 its cellular contents, and drawing the circlet of tentacles away from the 

 mouth, as illustrated in Figure 47. 



Haeckel assumed that the ancestral coelenterate was a polyp, because 

 of the ease with which a polyp can be derived (in theory) from the Gas- 

 trea. But study of what appears to be the most primitive order of hydroids, 

 the Trachylina, has led to the conclusion that the medusa phase is pri- 

 mary and the polyp derived. Once formed, the Coelenterata diverged 

 along three major lines, each comprising a class of the phylum. In the 

 most primitive class, the Hydrozoa, both generations are generally well 

 developed. The class includes hydroids such as the Hydra of elementary 

 laboratories and the much more typical marine colonial forms, such as 

 Ohelia. There are also some in which the polyp generation is reduced, as 

 in the order Trachylina. In the class Scyphozoa, including the jelly fishes, 



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