THE PHYLA HEMICHORDATA AND CHORDATA 



Class Pterobranchia. Small, sessile, or frce-Hoating hemichords with a crown of 

 tentacles and a U-shaped gut; often producing colonies bv budding, (.'ephalodisais, 

 Rhahdopleurn (Fig. 18.2). 



Class Balanoglossida. Worm-like hemichords, with body conspicuously divided 

 into proboscis, collar, and trunk regions. Saccoohssus, Bnlanoolossns, and others 

 (Fig. 18.1). 



PHYLUM CHORDATA 



DIVISION ACRANIATA. Without a skull, or cranium, and without vertebrae or 

 appendages. 



Subphylum Urochordata or Tunicata. Adults hardly recognizable as chordates. 

 Larvae, known as "tadpoles," with a notochord in the tail, with gill slits, and with 

 a dorsal, tubular central nervous system. During metamorphosis the notochord is 

 lost, the gill slits are modified into a pharyngeal basket, and the nerve cord is re- 

 duced to a ganglionic mass. Tunicates, or sea squirts, and many others (Figs. 18.3, 

 18.5). 



Subphylum Cephalochordata. With some resemblances to the tadpoles of 

 tunicates, and also to fishes; with persistent notochord and gill slits, with a per- 

 sistent dorsal, tubular nervous system, but without a skull or vertebrae. The 

 amphioxus or lancelet, Branchiosloma { = Amphioxiis), and others (Fig. 18.6). 



DIVISION CRANIATA. With a skull, or cranium, and usually with vertebrae and 

 paired appendages. 



Subphylum Vertebrata. Identical with Craniata: only subphylum in this 

 division. 



SUPERCLASS PISCES. With gills, and usually with appendages, commonly repre- 

 sented by two pairs of fins suited for locomotion in water. The various types 

 of fishes. 



Class Agnatha. Without true jaws and without limbs. Extinct forms, the 

 ostracoderms, a large group of primitive fishes bearing large scales. Existing forms, 

 the lampreys and hagfishes, without such scales (Fig. 18.9). 



Class Placodermi. Armored fishes of specialized type, extinct, and not closely 

 related to any existing forms. 



Class Chondrichthyes. Shark-like fishes, with exposed gill slits. Extinct forms 

 with bony skeletons, existing forms with cartilaginous skeletons. Sharks, rays, 

 skates, etc. (Fig. 18.10). 



Class Osteichthyes. With bony skeletons, and with gill slits covered externally 

 by an operculum. Include the lobe-finned fishes (mostly extinct), the numerous 

 existing ray-finned fishes, and the lungfishes. Common marine and fresh-water 

 fishes (Figs. 18.11, 18.12). 



SUPERCLASS TETRAPODA. With lungs, and usually with appendages which are 

 commonly represented by two pairs of limbs suited for locomotion on land. The 

 air-breathing vertebrates. 



Class Amphibia. Semiterrestrial and aquatic forms; cold-blooded; skin usually 

 soft and moist; respiratory exchange usually by gills in larva and by lungs and skin 

 in adult. Typically with eggs laid and developing in water. Salamanders, frogs, 

 toads, etc. 



Class Reptilia. Terrestrial or secondarily aquatic forms, covered with scales 

 or horny plates. Cold-blooded; lung-breathing; typically oviparous, some ovovivi- 

 parous. Turtles, lizards, snakes, alligators, etc. 



Class Aves. Terrestrial forms, usually adapted for flight, covered with 

 feathers. Warm-blooded, lung-breathing, oviparous. Birds. 



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