MINOR PHYLA 357 



Operculum 

 closed. 



Tentacles 



Mouth 



Ope.rculura 

 opened. 



Area, of 

 tuddind 



~Anae 



Stomach 



Retractor 

 muscle 



Figure 1 8.4. Diagrammatic view of two individuals in a colony of Bryozoa. The up- 

 per individual is retracted. (After Twenhofel and Schrock.) 



an assemblage of animals with lophophores. Bryo/oans also show some 

 similarities with the pterobranchs, a class in the phylum Hemithordata 

 (Chapter 19). It seems best to leave this phylum in an indefinite position 

 between the two major series ot eucoelomates. 



163. The Chaetognatha 



The Chaetognatha (Fig. 18.5) or arrowworms are a phylum of a few 

 species that may be extremely abundant in the marine plankton. These 

 small worms prey voraciously on other small animals, grasping them 

 with the anterior spines and s^vallowing them whole. They float mo- 

 tionless in the water and move in sudden jerks by flips of the body. 

 Arrowworms are transparent, revealing much of their internal anatomy 

 without dissection. They lack both circulatory and excretory systems, 

 but have a spacious coelom divided into a head cavity, a pair of trunk 

 cavities, and a pair of postanal tail cavities. The paired cavities are 

 separated by vertical mesenteries. The worms are hermaphroditic, with 

 ovaries in the trunk cavities and testes in the tail cavities. The nervous 

 system is composed of a well developed brain and a single large ventral 

 ganglion. 



Development is direct. The egg undergoes simple cleavage and the 

 coelom is enterocoelous. The mouth forms as a new opening consider- 

 ably in front of the blastopore. In its early embryology, therefore, the 

 arrowworm resembles the echinoderm-chordate series. In other respects, 

 however, they show no resemblance whatsoever. The phylum is usually 

 grouped with the echinoderm-chordate series, but it seems preferable to 

 place a gap between them. It is possible that this phylum evolved from 

 the same stock that produced the chordates, but diverged early and then 

 followed a somewhat parallel course of evolution. 



The minor phyla include animals of interest primarily to the zoolo- 



