DESCRIPTION OF THE PHYLUM CEPHALOCHORDATA (OWEN 1846)
| EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>CEPHALOCHORDATA |
PHYLUM CEPHALOCHORDATA LINKS
| Cephalochordata (se-fa-lo-kor-DA-ta) is derived from two Greek roots that mean “head cord” [head -kephali (κεφαλή); and cord -chordi (χορδή)]. The reference is to the notochord extending into the animal’s head. The name was coined by Owen (1846) and in most systems holds the rank of subphylum within the Chordata. |
| INTRODUCTION TO THE CEPHALOCHORDATA The lancet, also called amphioxus, is a free-swimming marine organism that bears a striking resemblance to the ammocetes larva of the lamprey. Generally the lancet lives tucked into the sediment exposing only its mouth (Figure 1), which takes in water to filter through their pharyngial (text with tooltip) The pharynx is the anterior part of the digestive tract, just behind the mouth. gill slits (Figure 2). The bodies are long and pointed at both ends (the name amphioxus mean pointed at the ends), and are a translucent pink to brown. They swim fish-style by the contractions of their segmented V-shaped myomeres and the laterally-compressed caudal flipper. In addition to the pharyngial gill slits, cephalochordates, as adults, have the diagnostic chordate characters: hollow dorsal nerve cord (text with tooltip) A hollow nerve cord develops from the the invagination of the ectoderm. and a notochord (text with tooltip) A notochord is a cartilaginous rod that lies ventral to the hollow dorsal nerve cord in the chordate phyla. , which extends from the tail forward into their rostellum (text with tooltip) A rostellum is a prominent rounded apex on the scolex of some tapeworms. It also is a beak-like tubular mouth part in some sucking insects. , well beyond the anterior ganglion. Unlike the craniates, they have no cranium, and the head is undifferentiated. Furthermore, they have no heart, but instead have pulsating blood vessels. Brusca and Brusca (2003) indicate that this group (a subphylum in their system) can be called Acrania (= without a head). Traditionally, the lancets have been considered the sister group to the vertebrates and comfortably within the clade of the chordates (e.g. Nielsen 2001 and Brusca and Brusca 2003; see Cladograms of the Deuterostome Phyla). However, that view has not been universally held. Putnam et al. (2008) examined the amphioxus genome relative to whole genomes of other deuterostomes and found that the cephalochordates were not sisters to the vertebrates, but basal in the whole chordate clade. The analysis of Delsuc et al. (2006) was even more surprising in that it concluded that the cephalochordates were associated with the echinoderms. If so, the cephalochordate body plan is primitive and the sessile natures of the echinoderms and tunicates are secondary simplifications. The molecular results call to question the concept of the chordate. In fact, the results of Delsuc et al. (2006) indicate that the “chordates” are paraphyletic. More recent work by Delsuc et al. (2008) and Edgecombe et al. (2011), however, confirm that the chordates (clade C in Figure 3) are monophyletic, but the Urochordata (tunicates) are sisters to the vertebrates. We follow Nielsen (2001) and Margulis and Schwartz (1998) in elevating the lancets to phylum-level status from the traditional Chordate subphylum designation. |
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| FIGURE 1. A living amphioxus in feeding position. Image from http://io.uwinnipeg.ca/~simmons/16cm05/1116/chordate.htm | FIGURE 2. Preserved and stained amphioxus to show pharynx with gill slits and notochord. Image from http://io.uwinnipeg.ca/~simmons/16cm05/1116/chordate.htm |
FIGURE 3. A cladogram of the phyla of the Deuterostomata with two major clades : Chordata (C) and Ambulacraria (A). Cephalochordata has a basal position in the chordate clade (C). The topology of this cladogram is a summary of Edgecombe et al. (2011) and Putnam et al. (2008).
| LITERATURE CITED Barnes, R. D. 1980. Invertebrate Zoology. Saunders College/Holt, Rinehart and Wilson, Philadelphia. Barnes. R. S. K. 1984a. Kingdom Animalia. IN: R. S. K. Barnes, ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc., Sunderland, MA. pp. 129-257. Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. 2nd Edition. Sinauer Associates. Sunderland, MA. Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago. Colbert, E. H. and M. Morales. 1991. Evolution of the Vertebrates, A History of the Backboned Animals Through Time. WILEY-LISS, Inc. New York. Delsuc, F., H. Brinkmann, D. Chourrout, and H. Philippe. 2006. Tunicates and not cephalochordates are the closest living relatives of vertebrates. Nature. 439: 965-968. Delsuc, F., G. Tsagkogeorga, N. Lartillot, and H. Philippe. 2008. Additional molecular support for the new chordate phylogeny. Genesis. 46: 592-604. Edgecombe, G. D., G. Giribet, C. W. Dunn, A. Hejnol,R. M. Kristensen, R. C. Neves, G. W. Rouse, K. Worsaae, and M. V. Sorensen. 2011. Higher-level metazoan relationships: recent progress and remaining questions. Organisms Diversity and Evolution. DOI 10.1007/s13127-011-0044-4. Hickman, C. P. 1973. Biology of the Invertebrates. The C. V. Mosby Company. Saint Louis. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York. Owen, R. 1846. Lectures on the Comparative Anatomy and Physiology of the Vertebrate Animals, Delivered at the Royal College of Surgeons of England, in 1844 and 1846. Pt. 1 Fishes. Longman, Brown, Green, and Longmans. London. Putnam, N. H., T. Butts, D. E. K. Ferrier, R. F. Furlong, U. Hellsten, T. Kawashima, M. Robinson-Rechavi, E. Shoguchi, A. Terry, J.-K. Yu, E. Benito-Gutiérrez, I. Dubchak, J. Garcia-Fernàndez, J. J. Gibson-Brown, I. V. Grigoriev, A. C. Horton, P. J. de Jong, J. Jurka, V. V. Kapitonov, Y. Kohara, Y. Kuroki, E. Lindquist, S. Lucas, K. Osoegawa, L. A. Pennacchio, A. A. Salamov, Y. Satou, T. Sauka-Spengler, J. Schmutz, T. Shin-I, A. Toyoda, M. Bronner-Fraser, A. Fujiyama, L. Z. Holland, P. W. H. Holland, N. Satoh, and D. S. Rokhsar. 2008. The amphioxus genome and the evolution of the chordate karyotype. Nature. 453: 1064-1071. Storer, T. I. and R. L. Usinger. 1965. General Zoology. 4th edition. McGraw-Hill, Inc. New York. Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 04/07/2013 |
