112 SUB-CLASS MARSIPOBRANCHII (CYCLOSTOMATA). 



Abdominal pores as distinct from the genital pores appear 

 to be absent. 



In Myxinidae the pronephros persists as a lobulated organ 

 in the pericardial cavity of the adult, and was called by Miiller 

 the suprarenal body. In Petromyzon it is quite absent in the 

 adult through present in the larva. 



The pronephros of Petromyzon is developed in the embryo and has three 

 or four body-cavity openings and a continuous glomerulus. It is in rela- 

 tion with the pericardium and atrophies during the Ammocoetes stage. 

 The kidney is developed in the young larva posteriorly, its anterior 

 end being a short distance behind the pronephros. The tubules arise 

 as excavations in the mesoblastic tissues. This larval kidney of the 

 Ammocoetes atrophies after the metamorphosis and is replaced by an 

 exactly similar structure placed further back. 



The pronephros of the adult Myxinoid consists of a large number of 

 nephrostomes which end blindly internally in a mass of lymphoid tissue, 

 or possibly in some cases, perhaps in young specimens, open internally 

 into an isolated anterior portion of the pronephric (segmental) duct. There 

 is also at the hind end of this organ a glomerulus of some size projecting 

 into an open recess of the pericardial cavity. 



The whole excretory system of Bdellostoma * appears to develop in the 

 same way in which the pronephros does in other types, that is to say the 

 longitudinal duct (pronephric) and the excretory tubules (segmental 

 tubules) arise in continuity with each other from the body-cavity epithe- 

 lium. The parts of the body-cavity into which the segmental tubes open 

 soon become separate from the rest and form a series of small vesicles each 

 communicating with a segmental tube. These vesicles become the malpi- 

 ghian bodies and the segmental tubes become the renal tubules. It is 

 not known how the pronephric part of the system acquires the peculiar 

 structure which it exhibits in the adult. 



As stated above the persistent kidneys of Marsipobranchs show no 

 differentiation into meso- and meta-nephros and the testes are not con- 

 nected with them. 



The generative organs are unpaired. They are attached to 

 the dorsal wall of the body cavity by a broad membrane, and 

 the generative products, both male and female, are shed into the 

 body cavity, whence they escape by the genital pores. Myxine 

 is hermaphrodite, and the reproductive gland produces sper- 

 matozoa before ova f (protandrous). Petromyzon is dioecious, 

 but ova have been observed in the testis. 



* Price, op. cit. 



f J. T. Cunningham, "Reproductive elements in Myxine glutinosa" 

 (Q. J. M. S., 27, 1887), and " Spermatogenesis in M yxine " (Q. J. M. S., 33, 

 1891). The hermaphroditism of Myxine is denied by Dean (Journ. Coll. 

 Sci., Tokyo, 19, 1904). 



