4 68 



CYCLOSTOMATA. 



out by their efferent tubes at a single aperture on each side. 

 The respiratory pouches have much plaited internal walls, 

 on which the blood vessels are spread out. On the left side, 

 behind the sixth pouch, a tube opens from the oesophagus 

 to the exhalent aperture. 



Vascular System. 



The blood contains nucleated red corpuscles. It is 

 collected from the body in anterior and posterior cardinals, 

 passes through a sinus venosus 

 into the auricle of the heart, 

 thence to the ventricle, thence 

 along a ventral aorta which 

 gives off arches to the respira- 

 tory pouches. From these the 

 purified blood passes dorsal- 

 wards in efferent branchial 

 vessels, which unite posteriorly 

 to form the dorsal aorta, 

 while from the most anterior 

 a branch goes to the head. 



FIG. 151. Respiratory Sys- 

 tem of Hag, from ventral sur- 

 face. 



g. Gullet; g.p.) first gill pouch; e.t., 

 exhalent tube of first gill pouch, unit- 

 ing with those from the other five 

 pouches ; #./., exhalent aperture ; v., 

 ventricle of heart ; v.a., ventral aorta, 

 carrying blood to gill pouches. 



Excretory System. 



The segmental or archi- 

 nephric ducts remain unsplit, 

 and the kidney or nephridial 

 system is represented by a 

 series of small segmental 

 tubules attached to the ducts. 

 The pronephros or fore kid- 

 ney persists, apart from the 

 functional mesonephros, in a degenerate state on each side 

 of the pericardium. The segmental ducts are said to end 

 much in the same way as they do in the lamprey. 



Reproductive System. 



Myxine is a protandrous hermaphrodite, spermatozoa 

 being formed at an early period, and ova afterwards. The 

 reproductive organ is simple, unpaired, and moored by a 

 median dorsal mesentery. Owing to the large size of the 

 ova, the ovary is very conspicuous in full grown forms. 



