THE MAIN LINES OF ANIMAL EVOLUTION 



among the Chordata, but the tunicates and amphibians have tadpole lar- 

 vae, while the cyclostomes have a unique larva, the Ammocoetes. 



Phylum Chaetognatha. The Chaetognatha are a small and uniform 

 group of marine worms, the arrow worms, which show little evidence of 

 relationship to any larger group of animals. They are included here be- 

 cause of conformity to the general deuterostome characters, yet they show 

 no more specific affinity to any of the remaining deuterostome phyla, and 

 it seems probable that the phylum branched off very soon after the forma- 

 tion of the deuterostome line. They show a superficial resemblance to 

 Amphioxus, but this is undoubtedly misleading. Aside from a few doubtful 

 specimens, the phylum is not represented in the fossil record. 



Phylum Pogonophora. The Pogonophora is a phylum of deep-sea 

 worms, only recently discovered and still but little known. The body con- 

 sists of small protosoma and mesosoma, and a very elongate metasoma. 

 The protosoma may bear one or more tentacles. It has an unpaired coelom, 

 drained by a pair of nephridial ducts. In the other body segments, the 

 coelom is paired. A nervous mass and ring in the protosoma give rise to 

 a paired dorsal nerve cord. The circulatory system consists of two longi- 

 tudinal vessels. Musculature is made up of subepidermal longitudinal 

 fibers. There is no digestive system whatever. Larvae are unknown. What 

 little is known of these animals seems to ally them with the Deuterostomia. 



Phylum Echinodermata. The Echinodermata are best known by the 

 starfishes, and these perhaps typify the phylum well. All echinoderms 

 have secondarily established radial symmetry in the adult, after beginning 

 life as a free swimming, bilaterally symmetrical larva. The radial sym- 

 metry of the adults is generally based upon a pentamerous (five radial 

 segments) plan, or upon a plan derived from such. The development of 

 radial symmetry may have been related to the change from a pelagic to 

 a sessile mode of life by primitive echinoderms, for radial symmetry is a 

 general characteristic of sessile organisms. The fossil record of the echino- 

 derms is one of the best, going clear back to early Cambrian times, and 

 excellent phylogenies can be constructed within each of the five extant 

 and two extinct classes. But the record does not throw light upon the 

 origin of the phylum, nor upon its possible relations to other phyla. The 

 questions depend, at present, entirely upon the embryological evidence, 

 with all of its limitations. 



Phylum Hemichordata and the Origin of the Chordata. The Hemi- 

 chordata are a small phylum of worm-like marine animals which have 

 been extensively studied because of their supposed relationship to the 

 Chordata. In Haeckel's phylogeny of the vertebrates, the hemichordates 

 were given as the next stage after the primitive flatworm. They were orig- 

 inally classed as a subphylum of the Chordata, because they show the 

 three basic diagnostic characters of the Chordata: a dorsal nerve tube, 

 a pharynx modified for respiration, and a notochord. Yet each of these is 

 equivocal. The dorsal nerve tube is confined to the collar region, and the 

 main nervous system is a ventral nerve cord like that of many inverte- 

 brates. The pharynx is pierced by numerous gill slits, yet it appears that 

 these function primarily as exit pores for the feeding current rather than 



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