AMPHIBIA. 403 



The heart lies anteriorly, as in Fishes, except in Gymnophiona. It con- 

 sists of a sinus venosus divided into a larger right moiety, receiving the 

 venae cavae, and a smaller left moiety receiving the pulmonary veins : of 

 two auricles separated by a septum, which is complete in Amtra but often 

 much fenestrated in Urodela, e. g. Proteus, Caecilia, Triton ; and of a 

 single ventricle. There is always a contractile conus arteriosus, as 

 in Dipnoi and Elasmobranchii, the muscles of which are striated. It is 

 separated usually by a row of three valves from the ventricle and of four 

 from the truncus aortae. One of the anterior valves is of great length 

 and takes a spiral course in the conus corresponding to the typical spiral 

 twist of that structure. This twist is sometimes lost. The number of 

 valves may be increased. In Caecilia (Gymnophiona) the conus is very 

 short, and there is but one row of valves, probably the distal. The truncus 

 aortae is short, especially in the Anura. A longitudinal septum crosses 

 it from side to side in such a way that it cuts off a ventral cavity, from 

 which the first or carotid arch, and the second or aortic arise, and a dorsal, 

 from which the third and fourth arches arise. These vessels are all closely 

 united at their origins. In the Tadpoles of both Urodela and Anura there 

 are four vascular arches. There is a direct anastomosis between the three 

 first branchial arteries and veins in Urodela, but a connection through the 

 branchial capillaries only in Anura. The fourth arch never supplies a gill, 

 but gives off the pulmonary artery. The first arch gives off the internal 

 and external carotids before it unites with the remaining three. At the 

 metamorphosis the first arch either retains its connection with the second 

 as in Triton and Derotremata, or loses it, as in Salamandra and Anura. The 

 second enlarges and becomes the aorta : the third dwindles, and is either 

 lost in Triton, Derotremata and Anura, or persists irregularly in Sala- 

 manders. The fourth arch enlarges and supplies the lungs : and in Anura 

 a large pulmo-cutaneous vessel arises close to the place where the third 

 arch was connected with it. The Perennibranchiata retain the condition 

 necessary to branchial respiration as seen in Urodela, but Menobranchus 

 and Proteus have no trace of the fourth arch except its anastomosis with 

 the third, which serves as the origin of the pulmonary artery, and Siren has 

 but a small rudiment of it springing from the third arch. There is a carotid 

 gland, so-called in Amir a, Salamandrina, and in Amphiuma alone among 

 Derotremata. It is absent in Perennibranchiata and Gymnophiona. It is 

 produced, as in the Frog, by the development of anastomoses between the 

 external carotid, i.e. lingual artery, and the first branchial artery. The aortae 

 unite below the backbone. In Anura the left arch gives off the caeliaco- 

 mesenteric artery before uniting with the right. Two superior venae cavae, an 

 inferior cava, epigastric vein and renal-portal system are always present, as 

 in the Frog (see pp. 76-7). There are subcutaneous lymph-spaces, of great 

 size and distinctness in Anura ; lymph-vessels in the skin, &c. ; a lymph 



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