304 



LOWER VERTEBRATES. 



the freedom of the embryo, in which there are structures for securing respiration in 

 the water. These gills differ from those of iishes, in that the fringes in which the 

 blood is agrated stand on fleshy processes of the branchial bones, and not directly on 



the bones themselves, though similar struc- 

 tures are found in the preliminary stages of 

 some fishes. During this stage the tail is 

 more or less modified as a swimming organ, 

 and the condition of the skull diffei-s materi- 

 ally in character from that of the adult. In 

 the tailless or anurous Batrachia, the limbs 

 do not appear until this period has nearly 

 closed ; while in the tailed or Urodele order, 

 the limbs appear almost immediately after 

 the gills. Besides these transitional charac- 

 ters, the Urodela possess in their early larval 

 condition a long process in front of the first 

 gill on each side, which is termed a balancer. 

 This remains in a few abnormal cases in 

 salamanders, but is permanent in the sub- 

 order of the Cascilians, or worm-like Batra- 

 chia. A similar process exists in the larva 

 of the frogs of the genus Xenopus, which 

 resembles superficially a silnroid fish ; but in 

 the Anura generally, the balancers are want- 

 ing. 



The gills in the Anura (frogs, toads, etc.,) 

 are soon concealed by a growth of the skin, 

 which leaves a small orifice for the discharge 

 of water from the pharyngeal cavity. In one 

 group of these animals this opening is on the 

 middle line below, but in the great majority 

 it is single and is situated on the left side. 



The eggs of Batrachia are always de- 

 posited in the water or in damp places. In 

 a few instances the young do not seek the 

 water, and in one species (^Salamandra atra) 

 they are born alive. 



Oeders I. - V. — Extinct. 



The class is divided into nine orders; 

 viz., the Ganocephala, Rhachitomi, Embo- 

 lomeri, Stegocephali, Proteida, Trachysto- 

 mata, Urodela, Gymnophiona, and Anura. 

 Of these the first four are extinct, and the 

 last five have living representatives. The 

 first two orders have the vertebral bodies represented by three segments each, a basal 

 intercentrum, and two lateral pleurocentra (Fig. 171). In the Ganocephala, the occi- 



FiG. 171.— Dorsal and lateral views of vertebral 

 eolumn of Eryops meyacephalus, one fourth natural 

 size. 



