THE PHYLUM CHORDATA 33 



Sub-Phylum I. Cephalochordata (Adelochorda). 



This includes but a single family of fish-like creatures, of 

 which there are about twelve species. The type form is 

 Am-phioxus (more correctly known as BrancMostomd). 



Sub-Phylum II. Urochordata. 



Order 1. Larvacea (Appendicularia), free-swimming forms 

 with permanent tail. 



Order 2. Ascidiacea (Tunicates or Sea-Squirts), fixed forms 

 without tail in the adult. 



Order 3. Thahacea (Salpians), free swimming forms with- 

 out tail in the adult. 



Sub-Phylum III. Hemichordata. 



Order 1. Enteropneusta, including worm-like forms such 



as Balanoglossus. 

 Order 2. Pterobranchiata, sessile, tube-dwelling forms — 



Cephaiodiscus and Rhabdopleura. 

 Order 3. Phoronidia, tubicolous forms — Phoronis. 



Sub-Phylum IV. Vertebrata (Craniata). 



Class 1. Cyclostomata (round mouth eels). 



Class 2. Pisces (true fishes with jaws). 



Class 3. Amphibia (vertebrates with aquatic larvae, but 



usually air-breathing in the adult condition). 

 Class 4. Reptiha (cold-blooded, air-breathing vertebrates). 

 Class 5. Aves (birds, feathered vertebrates). 

 Class 6. Mammalia (beasts or quadrupeds). 



SUB-PHYLUM I. CEPHALOCHORDATA 



The Cephalochordata are considered first because their claims to 

 vertebrate relationship are stronger than those of the other prover- 

 tebrate sub-phyla. They are rather small, marine, fish-like animals, 

 usually called "lancelets" on account of their sharply pointed ends. 

 Amphioxus was first described by Pallas in 1778. On account of its 

 resemblance to a slug it was given the name of Ldmax lanceolatus, the 

 implication being that it was a moUusk. In 1804 Costa, an Italian 

 naturaUst, redescribed it as a fish, alUed to the lampreys and hag- 

 fishes, and, because he erroneously diagnosed the oral tentacles or 



