404 



THE ANIMAL KINGDOM. 



sinus between the two lamellae of the mesentery, which is connected with 

 the lymph vessels and spaces of the stomach and viscera, and with a large 

 subvertebral lymph space (cisterna lymphatica magna). The Anura have 

 an anterior and posterior pair of lymph hearts ; the Urodela, like the Rep- 

 tilia, only a posterior pair 1 . They connect the lymph system with the 

 vascular. 



All Amphibia, with the exception of some Gymnophiona (?), pass 

 through a branchiate condition. The Tadpole possesses three pairs of 

 external ciliated gills attached near the dorsal ends of the three first 

 branchial arches. The most anterior of these gills is the largest : their 

 form varies. In the perennibranch Urodela they are retained, and are 

 either lobed (Menobranchus\ dendriform (Proteus, Siren), or fringed. In 

 the larvae of other Amphibia which lose the gills, they are more or less 

 lobed, or dendriform, as a rule. In the Anuran Notodelphys they are 

 bladder-like, as in Caecilia compressicauda among Gymnophiona. Epicrium 

 glutinosttm in the same order has three pairs of dendriform gills 2 . There 

 are four branchial slits, reduced to three in Siren, or to two between the 

 first, second and third branchial arches in Proteus and Menobranchus. The 

 adjacent edges of the arches in Urodela are furnished with interlocking 

 tooth-like processes, as in Fish, which become very much complicated in 

 the Anuran Tadpole. The external gills are soon lost in Anura and re- 

 placed by internal foliate gills. The gill-slits are also covered, as in the 

 larvae of Salamanders, by a membranous opercular fold, which reduces the 

 external apertures. They are eventually closed. In one group of Ichthy- 

 oidea, the Derotremata, the slit between the third and fourth branchial 

 arches is retained permanently after the shedding of the branchiae. This 

 event does not always take place at the same date in the life of the larva. 



The inlet from the pharynx to the lungs is guarded by two arytenoid 

 cartilages in Urodela, to which are added a circular cricoid cartilage in 

 Anura. There are both constrictor and dilator muscles acting on these 

 cartilages. There is a trachea of considerable length in Menopoma, Am- 

 phiuma and Gymnophiona,^^ bronchi in Pipa and Dactylethra\ the cartilage 

 supports of these tubes do not form rings. In other Amphibia the air pas- 

 sages are only rudimentarily represented. There are two lungs, ellipsoidal 

 sacs in Anura, more or less cylindrical in Salamanders. As to Perenni- 

 branchiata they are short in Menopoma, long in the others, and much 

 contracted for part of their length in Protetts. The left lung is rudimentary 

 in the snake-like Gymnophiona. The inner surface of the lungs is areolated, 

 but in Menobranchtis and Proteus it is smooth. A peritoneal fold suspends 



1 Many contractile lymph-hearts have lately been discovered in Salamandra maculosa and 

 Siredon (Axolotl) ; Weliky, Z. A. vii. 1884. So too in the Frog's tadpole, Id. Z. A. ix, 1886. 



2 External gills are not known to exist in other Gymnophiona. The young Epicriiim glutinosum, 

 after losing its gills on hatching, has two gill slits. Caecilia oxyura when young has a single pair of 

 slits ; C. recurvirostra has none. 



