CHORDATA. 



Bilaterally symmetrical ccelomate animals with a notochord, 

 dorsal and tubular central nervous system, and a pharynx per- 

 forated by branchial clefts (gill slits). 



Sub-phylum 1. Hemichorda. 



The notochord is poorly developed and restricted 

 to the anterior end of the body. 



Order 1. Enteropneusta. 



Worm-like, with numerous branchial clefts, a 

 straight intestine, and a terminal anus. Body 

 divided into three regions proboscis, collar, and 

 trunk. Development usually with a metamor- 

 phosis, the larva being known as a tornaria. 

 (Balanoglossus, Dolichoglossus.) 



Order 2. Pterobranchia. 



Tubicolous, with one pair of branchial clefts or 

 none, a U-shaped alimentary canal, and a dorsal 

 anus situated near the mouth. Proboscis flat- 

 tened ventrally into a large "buccal disc," its 

 base covered dorsally by the collar which is pro- 

 duced into two or more tentaculiferous arms. 

 Trunk short, prolonged into a stalk. Reproduc- 

 tion by budding occurs. (Cephalodiscus, Rhab- 

 dopleura.) 



Order 3. Phoronidea (doubtfully placed with the chordates). 

 Tubicolous, with gregarious habits. The body 

 ends in a plume of ciliated tentacles; the alimen- 

 tary canal is U shaped. There is a larva known 

 as actinotrocha. (Phoronis.) 

 Sub-phylum 2. Urochorda. 



The adult body is enclosed in a tunic or test, and 

 usually lacks a notochord; the central nervous 

 system is reduced to a simple ganglion. With an 

 atrial cavity and a pharynx perforated by from 



190 



