142 ZOOLOGY sect. 



opposite condition holds good, so that, while hermaphrodite, each 

 individual is either predominantly female or predominantly male. 

 The eggs of both genera are of great proportional size, and those 

 of Myxine are enclosed when laid in a horny shell bearing 

 numerous hooked processes at each pole : by means of these the 

 eggs are entangled together, and probably also attached to 

 seaweed. 



In Bdellostoma stouti, the only Myxinoid of which the develop- 

 ment is known, the eggs are elongated and cylindrical, and contain 

 a large quantity of food-yolk. The segmentation is meroblastic, 

 being confined to a germinal disc situated at one end of the 

 elongated egg. The blastoderm thus formed extends gradually 

 over the surface of the yolk, which it only completely encloses at a 

 late stage, when the gill-clefts are all formed. Bdellostoma differs 

 from Petromyzon and resembles the majority of the Craniata in 

 the mode of development of the central nervous system, which 

 is formed, not from a solid ectodermal keel, but from an open 

 medullary groove the lips of which bend inwards and unite to 

 form a medullary canal. 



4. — General Remarks. 



The Lampreys and Hags are undoubtedly the lowest of 

 craniate Vertebrata, but are in many respects so highly specialised 

 that it is a matter of great difificulty to determine their affinities 

 with the remaining classes. The structure of the vertebral 

 column and of the cranium are undoubtedly primitive in 

 the extreme ; but in the development of what may be called the 

 accessory portions of the skull, such as labial cartilages, they 

 show a singularly high degree of specialisation. The branchial 

 basket is quite sui generis, the theory that its vertical bars are 

 true branchial arches, displaced outwards during development, 

 being quite unproved. The absence of functional jaws is very 

 remarkable, seeing that in the remaining Craniata these structures 

 always bound the mouth at a period when the skull is in the 

 stage of development in which it remains permanently in Cyclo- 

 stomes : it is quite possible that their functionless condition may 

 be due to degeneration accompanying the evolution of a suctorial 

 mouth. The brain, in spite of its small size, is in some respects — 

 notably in the presence of cerebral hemispheres — of a more 

 advanced type than that of some of the true Fishes. The 

 circumstance that the pituitary pouch perforates the skull-floor 

 from above and becomes early associated with the olfactory sac, is 

 unique among the Vertebrata. The kidney of Bdellostoma is of 

 the most primitive type, and the presence of a large pronephros 

 is a significant archaic character. The total absence of limbs 

 may be a result of degeneration. ' ' ' -' 



