426 THE ANIMAL KINGDOM. 



dosteus, the two ductus in Polypterus, enter the auricle, not the sinus. 

 There is a hepatic-portal and a renal-portal system, the latter supplied by 

 the branches of the caudal vein. The hepatic veins enter the sinus venosus 

 independently, and there is no vena cava inferior. The lymphatic system 

 consists of vessels lodged in the myocommata and other fibrous septa, and 

 of spaces surrounding the chief vessels, and a right and left or a single sub- 

 vertebral canal. Masses of adenoid or lymphatic tissue imbed the kidneys 

 and the testes in Dipnoi, and are connected across the median line by 

 several bands. And in some Ganoids (Acipenser, Lepidosteus) and many 

 Teleostei the anterior part of the kidney is converted into a vascular 

 adenoid tissue. Lymphatic hearts, single in the Eel (Anguilla) or double 

 in Silurus, open into the caudal vein. The coelome opens externally by 

 abdominal pores in most Elasmobranchii (except four genera of Scyllidae, 

 Cestracion, Notidanidae, and Rhinidae\ in Holocephali, Ganoidei, some phy- 

 sostomous Teleostei (female Salmonidae, Mormyrus, Mnraenoidei], and the 

 Dipnoi. The two pores lie close to the anus in Holocephali, Ganoidei, 

 Salmonidae, and Mormyrus, close behind the cloaca in Ceratodus, at its 

 margin, or just within it, in Elasmobranckii. When double in Protopterus, 

 they open into the cloaca behind the rectum; when single, externally 

 and in front of it, and either within or without the limits of the sphincter 

 muscle. There is a single pore in Muraenoidei, which opens into the 

 ureter close to its external aperture. 



The thyroid gland is a large mass in Elasmobranchii, lying at the 

 anterior end of the ventral aorta : in Teleostei it is represented by masses 

 of reddish follicles lying ventrally to the same vessel, and scattered for a 

 greater or less distance along the branchial arteries. The thymus is 

 paired : each gland lies at the dorsal extremities of the branchial sacs in 

 Elasmobranchii : at the dorsal ends of the last pair of branchial arches close 

 beneath the mucous membrane of the branchial cavity in Ganoidei and 

 Teleostei. In the latter it is largest in half-grown individuals, and atrophies 

 in the adult. A spleen is always present. 



The pronephros seen in the embryo Atipenser, Lepidosteus, and 

 Teleostean appears to atrophy completely. The mesonephros forms the 

 permanent kidney in Ganoidei, Teleostei, and Dipnoi, but in Elasmobranchii 

 its hinder portion becomes specially enlarged, quite or nearly independent of 

 the fore-part, and acquiring ducts of its own, represents the metanephros of 

 higher Vertebrata. The anterior portion of the mesonephros is in this case 

 converted into epididymis in the male, parovarium in the female. The 

 size and form of the mesonephros are very variable. It is especially large in 

 chondrostean Ganoidei, and it extends in many Teleostei far into the 

 caudal canal. The two glands fuse anteriorly and posteriorly in chondro- 

 stean Ganoidei ; and in some Teleostei even for their whole length. In bony 

 Ganoidei and Dipnoi they are short, and restricted to the posterior part of 



