The Animal Kingdom - 665 



vertebrates evolved and how the vertebrate 

 classes are related to each other. But the 

 earlier record is very poor. Apparently early 

 chordates did not possess skeletal parts suit- 

 able for fossilization and we are left to specu- 

 late upon other clues including the study of 

 the few surviving lower chorda te forms. 



Subphylum h The Hemichordata. The acorn 

 worms, or tongue worms (Fig. 32-36), are the 

 principal representatives of the Hemichor- 

 data. Typically, such animals possess a stiff 

 tough proboscis (for burrowing), behind 

 which lie an acorn-shaped collar and an 

 elongate body (Fig. 32-36). Pharyngeal gill 

 slits are present in the anterior section of the 

 body, but the notochord is short, confined to 

 the proboscis region and not exactly similar, 

 structurally, to the notochords of other chor- 



Fig. 32-36. A hemichordate animal: the acorn worm 

 (tongue worm), Dolichoglossus. Perhaps the extinct 

 stock from which the hemichordates arose likewise 

 gave rise to vertebrates. (From General Zoology, by 

 Miller and Haub. Holt, Rinehart and Winston, Inc.) 



dates. Moreover, there is a ventral nerve cord 

 in addition to the dorsal one; and only the 

 anterior section of the dorsal nerve cord is 

 hollow. The larval stages of Echinodermata 

 and Hemichordata are very closely similar. 

 This may indicate origin from a common 

 early stock. 



Subphylum 2: The Urochordata. The Tuni- 

 cata (Fig. 32-37), which collectively comprise 

 the Urochordata, can be recognized as 

 Chordata only during the larval stages. The 

 larval tunicate (Fig. 32-37) is an active swim- 

 mer, which tends to resemble a good-sized 

 tadpole. Sectioned specimens display a dor- 

 sally placed, hollow nervous system, fairly 

 distinct gill slits, and a notochord. The noto- 

 chord is typical, except that it lies mainly in 

 the tail region. As adults, however, most 

 tunicates become attached to the bottom, 

 develop a tough muscular mantle or tunic 

 around the body, and lose all resemblance to 

 other Chordata, except that gill slits do 

 persist. 



Subphylum 3: The Cephalochordata. These 

 tiny fishlike chordates seldom reach a length 

 exceeding three inches. One common group 

 is called lancelets because of the slender, 

 tapered form (Fig. 32-38). For many years 

 they were also called Amphioxus, but more 

 recently the genus name Branchiostoma has 

 received priority. Lancelets are quite widely 

 distributed in tropical and semitropical 

 waters, along the Atlantic and Pacific coasts, 

 and in the Mediterranean area; and in one 

 locality, near Amoy on the southern coast of 

 China, they are caught by the ton and used 

 as food. 



As may be seen in Figure 15-10, the lancelet 

 shows the three unique chordate features in 

 unequivocal fashion. The dorsal hollow nerv- 

 ous system, gill slits, and notochord all are 

 typical, and all persist as functional struc- 

 tures in the adult animal. Moreover, these 

 animals display other features that indicate 

 a very close relationship to Vertebrata. These 

 include: (1) a segmental arrangement of the 

 muscles, (2) a segmental arrangement of the 

 spinal nerves, each with a dorsal and ventral 



