Development of the Kxcretory Organs of Hdellosfomf» stouti Lockin^ton. 207 



ov a little more of the duct, there being here one i);iir of tubules 

 for each body segment. The posterior part of the duct is entirely 

 free of tubules. Fürbringeu ('78, p. 39) states that in Myxine 

 oHstralis and Bdellosfoma hcferotremn the number of tubules equals 

 the number of myotomes. Embryology shows that this must be looked 

 upon as the more primitive condition. The tubules are straight, and 

 are so short that they look more like protuberance on the side of the 

 duct than like real tubules. In the blind, distal end of each tubule 

 there is a large Malpighian corpuscle. 



Johannes Müller ('42, p. 8), who gave an accurate description 

 of this part of the excretory system of the Myxinoids, called attention 

 to the fact that the Malpighian corpuscle has in every way the same 

 structure as the Malpighian corpuscle in the kidney of other verte- 

 brates. He recognized the fact that the Myxinoid kidney presents very 

 primitive characteristics, and said that it stood in the same relation 

 to the kidney of all other Vertebrates as do the mammary glands of 

 the Monotremata to the mammary glands of other Mammals, or the 

 liver of Amphioxus to the liver of other Vertebrates. This opinion 

 has since been somewhat modified, for the pronephros is considered 

 to be a more primitive structure than the mesonephros, and the organ 

 above described has been horaologized with the mesonephros of other 

 Vertebrates. But in the sense in which Müller meant it, his opinion 

 lias met with general acceptance, and some of his figures have been 

 widely copied, as illustrating the most primitive known mesonephros. 

 They are found in such well known books as Gegenbaur's and 

 Wiedersheim's Comparative Anatomies, and Balfour's and Hertwig's 

 Embryologies. 



The part of the excretory system which has been interpreted as 

 a pronephros lies on each side in the pericardial cavity, and just 

 posterior to the last gills. In an alcoholic specimen the organ on 

 one side was 8 mm long and 3 mm broad, and on the other, 6 mm 

 long and 2 mm broad , and there was an interval of 8 mm on the 

 one side, and 12 mm on the other, between the posterior end of the 

 pronephros, and the anterior end of the segmental duct. Weldon ('84) 

 has described the organ in Bdellostoma forsten. It is here larger than 

 in the California species, being from 20 to 25 mm long, and from 

 5 to 7 mm broad, but in the main features, at least, the structure 

 is the same in the two species. There is a main central duct, from 

 which a number of tubules are given off. Each of these tubules gives 

 rise by branching to a very great number of secondary tubules, which 



14* 



