CHAPTER 13 



Introduction to the 

 Higher Invertebrates 



The preceding chapters describe those animals usually referred to as the 

 "lower" invertebrates. These are lower in the sense that they lack some 

 of the structural complexity of the remaining or "higher" invertebrates, 

 and lower also in the sense that they are often thought to represent the 

 lower limbs of the evolutionary "tree." The metazoa are believed to have 

 evolved from the Protozoa, and the sponges, jellyfish and flatworms are 

 considered to be living representatives of groups that appeared early. 

 The roundworms and ribbon worms represent groups that arose some- 

 what later, possibly from the flatworms, and the higher invertebrates are 

 usually considered to have evolved still later. 



98. Evolutionary Relationships of the Sponges 



The sponges (phylum Porifera) are almost universally regarded as 

 the lowest group of metazoa, since they lack a nervous system as well as 

 excretory organs and a circulatory system, and in their organization 

 show a degree of independence among the cells that is not matched in 

 other metazoa. Since gastrulation in these animals is not readily com- 

 parable with that of other metazoa, and since their gastrodermis is also 

 not comparable, many authors conclude that the sponges may have 

 arisen separately from the Protozoa, or that they separated at a very 

 early time from other metazoa. To express this view the sponges are 

 often taken out of the metazoa and placed by themselves in the Parazoa. 

 That there is some degree of relationship between sponges and other 

 metazoa is suggested by the fact that their cleavages and processes of 

 gastrulation are to some extent comparable, and by the fact that the 

 sponges are, like other metazoa, diploid organisms with identical meiotic 

 processes in oogenesis and spermatogenesis. 



The similarity between sponge choanocytes and protozoan choano- 

 flagellates suggests that the sponges may have evolved from a choano- 

 flagellate-like ancestor. This is supported further by the tendencies in 

 living choanofiagellates to be colonial, to secrete "jelly," and to be en- 

 tirely holozoic. 

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