Survey of the Animal Kingdom 327 



although at certain stages the segments may be somewhat fused and difficult to 

 distinguish. Mouth and anus are present. Number of species of Chordata, 

 40,000. 



Classification of the Phylum Chordata 



Subphylum 1 — Hemichordata (hem i kor -da' ta) (Gr. hemi, half; chorde, 

 chord) or Enteropneusta (en ter op -nus' ta) (Gr. enteron, digt%\.\v& tvdict; pneuma, 

 breathe). — These marine, wormlike animals have a short, dorsal notochord in 

 the anterior end. Several pairs of permanent gill slits serve as respiratory organs, 

 with the internal gills. The anterior end of the body usually has a collar and a 

 fleshy proboscis. No cranium (brain case), jaws, vertebrae, or paired ap- 

 pendages. 



Arms 



--Tentacle 



Proboscis 



/Notochord 



CoUarnerve 



//iouth ^/^ 



tsophacfus. 



Ventral nerve 



TranU coe\om _ 



Trunk 



Intestine 



Fig. 140. — Rhahdopleura, a colonial chordate of the subphylum Hemichordata. 

 Enlarged, partially dissected, and somewhat diagrammatic. Actual length, 0.1 

 The collar nerve is also called the dorsal nerve. 



mm. 



Examples: Balanoglossus (Fig. 139), Cephalo discus, and Rhahdopleura (Fig. 

 140). 



Subphylum 2 — Urochordata (u ro kor -da' ta) (Gr. aura, tail; chorde, chord) 

 or Tunicata (tuni-ka'ta) (L. tunica, a mantle). — These marine animals have 

 small tadpolelike larvae with paired gill slits and both dorsal notochord and nerve 

 cord in the tail. In the adult stage the body may be tubular, globose, or irregu- 

 lar in shape (depending upon the species), covered with a transparent tunic which 

 is made of cellulose (a material common in plants). The adults are usually 

 sessile (attached), with many gill slits, but the notochord is usually absent and the 

 nervous system reduced. There are no cranium, jaws, vertebrae, or paired ap- 

 pendages. 



Examples: Ascidians and Appendicularians (Fig. 141). 



