Fill In the Blank: Use the below terms to complete the following paragraph.
Four major features distinguish the embryos of chordates from those of all other animals: a hollow dorsal (#4) _______ _________, a(n) (#5) __________ with slits in its wall, a(n) (#6) _________, and a tail that extends past the anus at least during part of their lives. In some chordates, the (#7) _____ chordates, the notochord is not divided into a skeletal column of separate, hard segments; in others, the (#8) _________, it is.
Invertebrate chordates living today are represented by tunicates and (#9) ______, which obtain their food by (#10) _____ ______; they draw in plankton-laden water through the mouth and pass it over sheets of mucus, which trop the particulate food before the water exits through the (#11) ______ _______ in the pharynx.
(#12) ________ are among the most primitive of all living chordates; when they are larvae, they look and swim like (#13) ______. A rod of stiffened tissue, the (#14) _______, runs much of the length of the larval body; it functions like a(n) (#15)_______ _____. Most adults remain attached to rocks and hard substrates. Even though the adult forms of hemichordates, echinoderms, and tunicates look very different, they have similar embryonic development patterns. Chordates may have developed form a mutated ancestral deuterostome (#16) ________ of an attached, filter-feeding adult. A larva is an immature, motile form of an organism, but if a (#17) ________ occurred that caused (#18) _____ _______ to become functional in the larval body, then a motile larva that could reproduce would have been more successful in finding food and avoiding (#19) _________ than an attached adult; in time, the attached stages in the species would be eliminated.
The ancestors of the vertebrate line may have been mutated forms of their closest relatives, the (#20) ____; in vertebrate ancestors, the notochord became segmented and the segments became hardened (#21) _____. The vertebral column was the foundation for fast-moving (#22) _____, some of which were ancestral to all other vertebrates. The evolution of (#23) ______ intensified the competition for prey and the competition to avoid being preyed upon; animals in which mutations expanded the nerve cord into a (#24) _____ that enabled the animal to compete effectively survived more frequently than their duller-witted fellows and passed along their genes into the next generations. Fins became (#25) ____; in some fishes, those became (#26) _____ and equipped with skeletal supports; these forms set the stage for the development of legs, arms, and wings in later groups. As the ancestors of land vertebrates began spending less time immersed and more of their lives exposed to air, use of gills declined and (#27)_____ evolved; more elaborate and efficient (#28) ____ systems evolved along with more complex and efficient lungs.
Terms to be used above (some may be used more than once; all will be used):
G. nerve cord
I. Tunicates (sea squirts)
K. gill slits
M. torsion bar
W. sex organs