THE PHYLA HEMICHORDATA AND ECHINODERMATA 3gl 



pouch; (3) the development of the adult madreporite from the pore of 

 the anterior pouch; (4) the loss of the preoral region; and (5) the develop- 

 ment of lower and upper body cavities from the left and right posterior 

 pouches. 



Thus, if a comparison between the hemichordates and echinoderms 

 is valid, the proboscis pore is homologous with the madreporite, the 

 collar cavity with the ring canal and radial canals, and the trunk cavity 

 with the echinoderm body cavity. 



Relationships of the hemichordates and echinoderms with phyla 

 previously considered are difficult to establish. Like many of the phyla 

 they have bottom-living adults and planktonic larvae but the larvae 

 may have evolved independently in several different lines to serve as a 

 dispersing mechanism. 



In all of the preceding phyla with a separate mouth and anus the 

 blastopore of the gastrula tends to become a ventral portion of the adult. 

 Usually it becomes elongated as it closes, and forms an antero-ventral 

 mouth at its anterior end. Typically an anus forms from its posterior end. 

 Numerous exceptions occur in which mouth, anus or both form from 

 tissue beyond the ends of the elongated blastopore. Those phyla in which 

 the mouth is clearly a part of the blastopore are called protostomous. 

 In the hemichordates and echinoderms (and possibly in some of the 

 minor phyla previously discussed) the fate of the blastopore is decidedly 

 different. The blastopore is posterior and closes without elongation to 

 form the anus, while the mouth forms a considerable distance away. It 

 is possible that this difference is so fundamental that the evolutionary 

 relationships between the protostomes and the deuterostomes will never 

 be discovered. 



In both the hemichordates and echinoderms the coelom is entero- 

 coelous, whereas in many of the previously considered groups it is 

 schizocoelous. In some, however, an enterocoelous origin is the rule, so 

 that a relationship between the hemichordates and such groups as the 

 Chaetognatha and some of the Brachiopoda may be indicated. On the 

 other hand, exceptions for the origin of the coelom are becoming so 

 numerous as more species in each phylum are studied that whether it is 

 enterocoelous or schizocoelous may not prove to be a very useful char- 

 acteristic. 



At the present time the Chaetognatha, which are enterocoelous and 

 in which the blastopore forms the anus and extends only halfway up 

 the ventral surface, are generally considered to be the closest of the 

 previously considered phyla to the Hemichordata and Echinodermata. 



The chordates, like the hemichordates and echinoderms, are clearly 

 deuterostomous. The fate of the chordate blastopore is not like that of 

 the hemichordates and echinoderms. As the chordate blastopore closes the 

 lips are drawn together dorsally and elongated, with the anus forming 

 from the posterior end. Such a pattern, while different from that of the 

 hemichordates and echinoderms, is more easily related to theirs than to 

 the protostomous pattern. 



Chordates are related to the hemichordates through a comparison 

 of the adults. Both groups have gill slits, and in some members of both 



