CHAETOGNATHA 

 THROUGH LOWER 

 CHORDATA 



Invertebrate Eucoelomates 



The remaining phyla in the animal kingdom are 

 called the Enterocoela (Gk. enteron, intestine + koilos, 

 hollow) because their coeloms form from pouches that 

 grow from the embryonic gut. Recall that in the 

 schizocoels the true body cavity develops from a split 

 in the mesoderm. The enterocoels include the Echi- 

 nodermata, Chaetognatha, Hemichordata, Pogono- 

 phora, and Chordata. 



RADIAL ENTEROCOELA: 

 ECHINODERMATA 



Having discussed the annelid-arthropod line, we 

 now turn to another such evolutionary group, the 

 echinoderm-chordate line. It was not implied in the 

 earlier discussion that the arthropod arose from the 

 annelid, but rather from an annelid-like ancestor 

 which gave rise to both annelids and arthropods. In 

 the same way, echinoderms, hemichordates, and 

 chordates probably arose from a common ancestor 

 that was not a member of any of these groups. The 

 relationship of the three phyla is best seen in larval 

 forms. Almost identical larvae occur in echinoderms 

 and hemichordates, and it is a short step from hemi- 

 chordate to tunicate larvae. Echinoderm-hemi- 

 chordate similarity is so marked that it fooled the 

 zoologist who first discovered a hemichordate larva; 

 he called it a starfish larva. 



The positions of chaetognaths and pogonophores 

 are less definite, but each group likely is a member of 

 the echinoderm-chordate line. 



ECHINODERMATA (Spiny-skinned Animals) 



Diagnosis: symmetry radial or biradial, a few bi- 

 lateral; unsegmented; enterocoelous; includes the 

 starfishes, brittle stars, sea urchins, sand dollars, sea 

 lilies, and sea cucumbers; all marine. 



It seems that the echinoderms must have evolved 

 through a hypothetical, free-swimming larva-type of 

 ancestor known as the dipleurula. Superficially this 

 larval stage resembled the trochophore; however, 

 both its mesoderm and coelom arose in a manner 

 different from that occurring in the trochophore. In 

 fact, the dipleurula probably looked and developed 

 much like living echinoderm larvae, but the dipleu- 

 rula could not have been exactly like any living 

 echinoderm larva. 



In surveying the hypothetical dipleurula, we must 

 examine certain features. It most likely had bilateral 

 symmetry, yet modern adult echinoderms are mostly 

 radially symmetrical. This implies that there must be 

 a metamorphosis in the development of living echino- 

 derm adults from their larvae. 



Perhaps this need for change can better be ex- 

 plained by a second hypothetical stage in echinoderm 

 evolution, the penlactula. Supposedly derived from the 



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