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Chordates, Protochordates, 

 and their Relationships 



THE PHYLUM CHORDATA 



The Phylum Chordata unites a number of seemingly dis- 

 tinct groups of organisms, one of which is the Subphylum 

 Vertebrata. The student already has some familiarity with 

 the vertebrates and has perhaps been introduced to the other 

 subphyla, at least by name. These subphyla are the Cepha- 

 lochordata and the Urochordata or Tunicata, which are 

 invertebrates, that is, without vertebrae. The fact that these 

 invertebrates are included with the vertebrates in a common 

 phylum is of special interest, for it suggests the possibility of 

 intergradation between these two extremes and, therefore, 

 the pathway for evolutionary transition from one to the 

 other. 



The historical background of this association is pertinent. 

 Lamarck (1809, 1815) and Cuvier (1816) created the schism 

 between the categories of vertebrates and invertebrates on 

 the basis of the uniqueness of the former, the presence of a 

 vertebral column. This subjective evaluation was not revised 

 even after the recognition by Costa in 1834 oi Branchiostoma 

 lanceolatiim, named Amphioxus by Yarrel a year later, as a 

 "low vertebrate," and the demonstration by Kowalevski 

 ( 1869, 1871 ) of the similarities in basic structure of verte- 

 brates and the larvae of tunicates. Balfour in 1880 destroyed 

 this schism, in theory if not in fact, by establishing the 

 Phylum Chordata for the vertebrates, lancets (Amphioxus), 

 and tunicates. Bateson in 1885 added the hemichordates, 

 containing Batanoglossus, to the Phylum Chordata. Fowler 

 (1892) suggested the inclusion oi Rhahdopleura in the Hemi- 

 chordata, and Spengel (1932) added Planctosphaera to this 

 subphylum. Current opinion removes the Hemichordata as 

 a separate phylum, leaving the Cephalochordata, Urochor- 

 data, and Vertebrata as subphyla of the Chordata. Collec- 

 tively, the cephalochordates, urochordates, and hemichor- 

 dates are identified as protochordates, thus inferring a 

 transitional nature between the lower invertebrate phyla 

 and the vertebrates. 



The Phylum Chordata as now defined unites organisms 

 having a notochord, hollow dorsal nerve cord, and pharyn- 

 geal pouches or slits. As a basis for discussion, the anatomy 

 of the various subphyla of the chordates and the hemichor- 



dates should be described. From these anatomical facts the 

 interrelationships of the chordate subphyla and the relation- 

 ships of chordates and hemichordates can be defined and 

 discussed. No specific order will be followed for this descrip- 

 tion, for all of these early animal forms are equally complex 

 in structure and none can be identified as ancestral or tran- 

 sitional to the others. Furthermore, to avoid anticipating the 

 results of comparative study and indoctrinating the student, 

 no specific order based on the conclusions of "authorities" 

 will be followed. It is recommended that the student work 

 from the raw materials of dissection and observation to con- 

 clusions regarding interrelationships of an evolutionary or 

 phylogenetic nature. 



Amphioxus is usually described as being most like the 

 vertebrate and is the protochordate that is perhaps best 

 known to the student. It thus constitutes a starting point in 

 working from the familiar toward the unknown. 



THE PROTOCHORDATES 

 Cephalochordata 



Bramhiostumu (B. lanceolatus or Amphioxus) is the usual 

 representative of this group, which has only one other genus 

 in it, Asymmelriin. The group derives its name from the fact 

 that the notochord extends farther forward than the brain 

 (in the vertebrate the cord ends about midway along the 

 length of the brain). 



The body is elongate and fusiform, or lanceolate (Figures 

 1-1, 1-5). .At the anterior end, the mouth lies within an oral 

 funnel or hood, below the snout or rostrum (Figure 1-2). 

 The membranous lateral and ventral walls of the oral hood 

 are fringed by cirri. These cirri have small lumps or sensory 

 papillae on them and, along with the margin of the oral 

 funnel, are supported by a jointed connective tissue skeleton. 



The mouth is set in the middle of a partition or velum, 

 and from the edge of the mouth tentacle-like languets ex- 

 tend into the pharynx. In the oral hood there is a ciliated 

 wheel organ and in the roof is a ciliated fossa (literally 

 "ditch") of Hatschek, which lies just to the left of the 

 notochord (Figure 1-3). 



The snout is somewhat expanded and into it extends the 

 notochord. The notochord extends the entire length of the 



