LOWER ANIMALS 



197 



\s^\j,l,L444^^ «"'"3^Tj"t**k 



Figure 12.4 Sponge types. Class Calcareo: 1, Rhabdodermetia, a vase sponge; 2, ieuconia, a 

 bristle sponge; 3, teucoso/en;a, an antler sponge. Class Hexoctinellido: 4, Hyalonema, a gloss rope 

 sponge; 5, Sfouroco/yptus, a gloss goblet sponge; 6, Eupiectella, Venus's flower bosket. Class Demo- 

 spongio: 7, Poterion, Neptune's goblet; 8, Chalina, a finger sponge; 9, Spongilla, a fresh- 

 water sponge; 10, Haticlona, on encrusting sponge. {4, 5, and 7 modifred from various sources.) 



group Eumetazoa implies that all its phyla have a 

 common ancestry. 



The relationships within the coelenterates is be- 

 lieved very important in the ancestry of Eumetazoa. 

 First, and because of their ciliated larva, it is often 

 assumed that the coelenterates originated from a cil- 

 iated or perhaps flagellated ancestral protozoan. Ac- 

 cording to this hypothesis, the "first" coelenterate 

 was a primitive medusan (jellyfish). This jellyfish, 



in turn, is considered the ancestor of all coelenterates. 

 From this stem jellyfish, and quite early, were derived 

 a hydromedusan-like animal that was probably close 

 to being the ancestor of all coelenterates and, also, 

 of all Eumetazoa. This assumption is based on such 

 things as hydromedusan structure resembling a 

 developmental stage of the Eumetazoa, for example, 

 hydromedusan tissues contain most of the cell types 

 found in Eumetazoa. Moreover, although coelenter- 



