124 VERTEBRATES WITHOUT JAWS iv. 19- 



The thyroid gland consists of a long series of sacs formed by 

 evagination from the floor of the pharynx. 



Down the sides of the body are pairs of slime glands, able to secrete 

 large amounts of mucus, which may be protective and is said also to 

 be produced under the operculum to hasten the end of a dying fish 

 that the hag has attacked. 



A curious difference from the nervous system of lampreys is that 

 the dorsal and ventral roots join, though the details suggest that the 

 union is not similar to that found in gnathostome vertebrates. The 

 brain shows several features of reduction and simplification and no 

 pineal eyes are present. There is only one semicircular canal in the ear 

 (p. 109). The kidneys show a more generalized condition than in any 

 other vertebrate in that the pronephros persists in the adult and is 

 hardly marked off from the mesonephros, so that an almost continu- 

 ous series of funnels and glomeruli can be recognized. Moreover, there 

 is a regular series of mesonephric glomeruli, a pair in each segment. 



The development is known only in Bdellostoma, where the egg is 

 yolky and cleavage partial, leading to the formation of an embryo 

 perched on a mass of yolk. It is often stated that Myxine is a protandric 

 hermaphrodite, because individuals are found in which the front end 

 of the gonad contains eggs, whereas the hind part is testis-like (Fig. 

 81). No ripe sperms have ever been found in this region, however, 

 and, moreover, individuals with fully testicular gonads do occur. 

 Since it is known that in other vertebrates (including the lampreys) 

 the gonads go through a hermaphrodite stage during development it 

 seems likely that Myxine is not a functional hermaphrodite but that 

 the double-sexed gonad shows a rather late persistence of the indeter- 

 minate stage. 



The hag-fishes all live in the sea and their blood differs from that of 

 other chordates in that it is isosmotic with sea water. However, the 

 individual ions are regulated; sodium and phosphate exceed their 

 values in sea water, and the other ions are present in lower concentra- 

 tion. It is usually assumed that fishes, with their glomerular kidneys, 

 evolved in fresh water. However, the very earliest fragments of 

 armoured agnathans are from Ordovician deposits that may be littoral 

 or marine and it might be that the condition of the blood and kidney 

 of Myxine is that of the earliest agnathans and that the glomerulus was 

 not evolved as an adaptation to freshwater life, as is often supposed 

 (Robertson, 1954). 



The organization of the lampreys and hag-fishes shows that they 

 preserve many characteristics from a very early stage of chordate 



