THE ANIMAL CLIMAX-THE CHORDATES 



283 



been found in comparing the embryos of 

 the echinoderm (bipinnaria) with that of 

 the acorn worm (tornaria) (Fig. 13-4). In 

 fact, they are so similar that it is very dif- 

 ficult to distinguish between them. They 

 are simple, bilaterally symmetrical, free- 

 swimming forms. The bipinnaria sits down 

 and develops radial symmetry to become an 

 echinoderm, whereas the tornaria grows 

 into the acorn worm. Still further back it 

 is generally believed now that the coelen- 

 terate type gave rise to three great groups: 

 the echinoderm-chordate stock, the flat- 

 worm stock, and the annelid-arthropod- 

 mollusk stock (Fig. 13-5). 



THE PROCHORDATES 



The acorn worm (sub-phylum Hemi- 

 chordata ) ( Fig. 13-3 ) is here considered 

 as the first member of the phylum Chor- 

 data, although some zoologists believe it 

 so divergent as to warrant a phylum by 

 itself. This earthworm-like animal lives 

 buried in the mud, using a protrusible pro- 

 boscis to move about as it feeds on organic 

 matter. It fulfills the required characteris- 

 tics of chordates by the presence of numer- 

 ous gill slits, a dorsal as well as a ventral 

 nerve cord, and a small anteriorly located 

 notochord. There are only a few (60) spe- 

 cies in the world, but individuals are rather 

 common on both the Atlantic and Pacific 

 coasts of the United States. Although they 

 have no apparent economic significance to 

 man, they are of interest to the zoologist. 



The tunicates, or sea squirts (sub-phy- 

 lum Urochordata ) (Fig. 13-6), are also 

 grouped with the chordates, although after 

 looking at the adult form one would 

 scarcely expect them to be classified here. 

 Commonly attached to rocks along the sea- 

 shores, they live by forcing water in and 

 out of their sac-like bodies through siphons, 

 resembling the clam in this respect. The 

 water passes into a large perforated phar- 

 ynx which strains out tlie tiny food particles 

 that are carried into the digestive tract. 



Gills line the many openings in the pharynx 

 wall, but aside from this one chordate char- 

 acteristic, it appears to have no claim to 

 membership among the chordates. 



However, a careful look at the larval 

 form demonstrates at once its true chordate 

 relationships, for the larva possesses a noto- 

 chord and dorsal tubular nerve cord, in 

 addition to the gill slits. As an embryo, the 

 animal is tadpole-shaped and swims ac- 

 tively in the sea water ( Fig. 13-6 ) . Late in 

 embryonic life, however, it settles on a rock 

 and metamorphoses into the sessile adult, 

 which is a degenerate form compared to 

 the active, free-swimming, fish-like chor- 

 date from which it came. Tunicates are 

 very numerous in the oceans of the world 

 and range from microscopic size to more 

 than 12 inches in diameter. They may live 

 in shallow or deep water and are commonly 

 found by the bather who is sufficiently curi- 

 ous to examine the rocks along the coast. 

 The group as a whole has no economic 

 significance. 



There is another tiny animal (2 inches 

 long ) that cannot be mistaken for anything 

 but a primitive chordate and, moreover, it 

 possesses body structures that force us to 

 believe that some such form might have 

 given rise to the vertebrates. This animal is 

 known as lancelet, or amphioxus ( sub-phy- 

 lum Cephalochordata ) (Fig. 13-7). It is 

 an ocean dweller, found in relatively few 

 though widely separated regions, and 

 reaches such numbers along a part of the 

 shore of China that it is utilized as a source 

 of food. Not only does it possess the three 

 chordate characteristics exhibited by the 

 two preceding groups, but it also has a 

 body plan that closely resembles that of 

 the vertebrates. 



Amphioxus has a general shape not un- 

 like that of a slender fish, with two longi- 

 tudinal folds of skin extending throughout 

 most of its length, which may be forerun- 

 ners of appendages (Fig. 13-8). Its noto- 

 chord functions as a semi-rigid supporting 

 internal skeleton, extending from one end 



