J32 



ZOOLOGY 



SECT. 



crtel^s 



Fir;. 7i3. Auditory organ of Myxine. 

 ii.ii-i.jt., amp.' ampullae; tn>i. .<. eiidolym- 

 phatic sac ; .. c. semicircular canal ; i<t,: 

 fir. utriculo-sacculus. (After Retzius.) 



nephric duct, Fig. 764, a) sends off in each segment a coiled tubule 



(b) or nephridium, with a single 

 Malpighian capsule (c), into which 

 a branch from the aorta (d] 

 enters and forms a glomerulus. 



Myxine is hermaphrodite and 

 protandrous i.e., the gonad of 

 the young animal produces 

 sperms, and at a later period 

 gives rise to ova. The eggs of 

 both genera are of great pro- 

 portional size, and those of 

 Myxine are enclosed when laid 

 in a horny shell bearing numer- 

 ous hooked processes at each pole : by means of these the eggs 

 are entangled together, and probably 

 also attached to seaweed. The develop- 

 ment of the Hags is not known. 



4. GENERAL KEMARKS. 



The Lampreys and Hags are undoubt- 

 edly the lowest of craniate Vertebrata, 

 but are in many respects so highly 

 specialised that it is a matter of great 

 difficulty to determine their affinities 



(/ 



with the remaining classes. The struc- 

 ture of the vertebra] column and of the 

 cranium are undoubtedly primitive in 

 the extreme ; but in the development of 

 what may be called the accessory por- 

 tions of the skull, such as labial cartilages, 

 they show a singularly high degree of 

 specialisation. The branchial basket is 

 <|uite sui generis, the theory that its ver- 

 tical bars are true branchial arches, dis- 

 placed 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 perma- 

 nently in Cyclostomes : it is quite pos- 

 sible that their functionless condition 

 may be due to degeneration accompany- 

 ing the evolution of a suctorial mouth. 



FIG. 764. A, portion of kidney of 

 Bdellostoma. B, segment 

 of same, highly magnified, a, 

 ureter ; b, urinary tubule ; f , 

 Malpighian capsule ; </, afferent 

 artery ; e, efferent artery. 

 (From Gegenbaur's Comparative 

 Anatomy. 



