GENERAL ZOOLOGY 



enterocoelous development indicates a position somewhere near the evolution- 

 ary stem which culminated in the Chordata. Members of the genus Sagitta 

 (Fig. 12.16) are pelagic forms, often very abundant in the marine plankton, 

 swimming by means of body movements and horizontal fins. There are 

 prehensile mouth parts consisting of a series of stout, chitinous hooks, sug- 

 gesting the name Chaetognatha, or "bristle jaws." A relatively large coelom 

 is divided transversely into three compartments by septa, and there are also 

 dorsal and ventral mesenteries linking the gut wall and the body wall. The 

 nervous system consists of a dorsal ganglion, from which nerve cords extend 

 as a circumpharyngeal ring to a ventral ganglion, with nerve fibers radiating 

 to all parts of the body. The animals are monoecious, and the zygotes 

 develop into miniature adults without a ciliated larval stage. 



Summary 



In this chapter we have discussed several groups of metazoan animals 

 which have little in common except their triploblastic, bilateral organization. 

 Each of these groups, with the exception of the Aschelminthes, presents a 

 consistent pattern of general characteristics sufficiently difTerent from those 

 of all other animals to warrant their status as separate phyla. There re- 

 mains some doubt among zoologists about the validity of erecting a phylum 

 Aschelminthes to contain the widely difTerent forms proposed for inclusion, 

 of which we have discussed only two; the pseudocoelous body cavity is their 

 chief unifying characteristic. 



The members of each of the phyla considered in this chapter have under- 

 gone extensive modification in their evolution, in adaptation to particular 

 ways of life or conditions in their environments. As evidence of their evolu- 

 tionary histories and phylogenetic affinities there remain only the most general 

 and fundamental characteristics. These enable us to determine the approxi- 

 mate level of organization which each type represents but not to state with 

 any confidence the true relationships of any of these groups. We may con- 

 clude that although they are interesting from the biological standpoint, per- 

 haps none of them has been of great significance in relation to the evolution 

 of the major phyla of animals. 



In the chapters to follow we shall discuss the major invertebrate groups — 

 phyla which have been successful, as judged from the numbers of species and 

 of individual organisms they contain. Generally speaking, these major groups 

 retain sufficiently significant characteristics to permit us to establish, with 

 some probability, their phylogenetic relationships. 



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