PETROMYZON. 473 



blood passes dorsalwards in efferent branchial vessels, which 

 unite posteriorly to form the dorsal aorta, while from the 

 most anterior a branch goes to the head. 



Excretory system. — The segmental or archinephric ducts remain 

 unsplit, but the morphological nature of the kidney of the adult is still a 

 matter of dispute. In the young form the pronephros is functional, but 

 at maturity the whole of it, or, according to another interpretation, the 

 anterior region only, degenerates into a lymphoid structure lying beside 

 the pericardium, and the mesonephros becomes functional. According 

 to Maas, however, its tubules are exceedingly simple in structure, con- 

 sisting of little more than glomeruli ; and the segmental duct, with 

 which the posterior portion of the pronephros is fused, is an excretory as 

 well as a conducting tube. In other words, the posterior portion of the 

 pronephros is functional throughout life. 



The unsplit segmental ducts end by separate pores on a papilla 

 within the integumentary cloaca. 



Eeproductive system. — Myxine is a protandrous herma- 

 phrodite, spermatozoa being formed at an early period, 

 and ova afterwards. The reproductive organ is simple, 

 unpaired, and moored by a median dorsal fold of peri- 

 toneum. Owing to the large size of the ova, the ovary is 

 very conspicuous in full-grown forms. When the ova are 

 freed from the ovary, they pass into the body cavity. Each 

 has an oval horny membrane, with a circlet of knobbed 

 processes at each end. By these they become entangled 

 together. There are no genital ducts, but just above the 

 anus there is a large genital pore opening from the body 

 cavity into the integumentary cloaca. The development 

 is still unknown. 



Besides Myxine glutinosa, two other species are known — one from 

 Japan, another from the Magellan Straits. The genus Bdellostoma, 

 from the Pacific coasts of America, off the Cape of Good Hope, etc. , is 

 nearly allied. 



The best-known species, Bdellostoma dombeyi, resembles the hag in 

 many ways. It lives at the bottom of the sea, at depths of a hundred 

 fathoms or more, and is often found inside caught halibut, etc. The 

 gill-pouches have separate openings, and are extraordinarily variable in 

 number, from six to fourteen on either side — a variability perhaps point- 

 ing to ancestral reduction from a larger number (cf. Amphioxus). Ayers' 

 experiments show that the removal of one or both ears in this form does 

 not materially affect equilibration. 



