HABITS, AFFINITIES. 



115 



a very complicated glandular form. After the metamorphosis 

 it is less conspicuous and appears to be without the opening 

 into the throat. 



In M tjxinidae the eggs are very much larger (19 mm. X 7 mm. 

 in Myxine, 31 mm. x 9-5 mm. in Bdellostoina) and contain a 

 considerable quantity of yolk. They are enclosed in a horny 

 case with hooked processes proceeding in tufts from each end. 

 The egg case appears to be the vitelline membrane. 



The Ammocoete lives buried in mud and sand and likes 

 dark places. It lives on small aquatic organisms {Infusoria^ 

 Daphnia, Rotifers, etc.). The marine lamprey ascends rivers 

 at spawning time, sometimes carried by the salmon or shad 

 {Alausa vulgaris), to spawn. They eat worms and small aquatic 

 animals. The Myxinidae live exclusively on other fishes. They 

 are able by their 

 formidable dental 

 armature and 

 powerful lingual 

 muscles to make 

 their way into the 

 body cavity of 

 their prey, in which 

 they are sometimes 

 found embedded. 



The Marsipo- 

 branchii are sometimes spoken of as a degenerate group. 

 We do not think that there is any evidence of degenera- 

 tion. The most important points in which they differ from other 

 fishes relate to the skeleton, and to the nasal organ. But these 

 are precisely the organs which show the greatest amount of 

 variation within the group as at present constituted. This 

 seems to point to the fact that they separated off from other 

 fishes at a time when these two organs were in a highly indeter- 

 minate condition, and had not attained to that fixity of structure 

 which characterises on the whole the general arrangement of the 

 skeleton, and nasal and pituitary sacs in other fishes. 



The condition of the eyes in Myxinidae might be held to be evidence of 

 degeneration, but we should rather be inchned to regard it again as the 

 survival from a time when the visual organ was more variable and had 

 not obtained that fixity of character which it has at the present day. No 



U Vc 



Fio. 60. — Diagrammatic longitudinal section tlirougii the 

 head of a larva of Petromyzon (after Balfour). Ah optic 

 vesicle ; C heart ; Ch notochord ; H thyroid involution ; 

 Ks branchial pouches ; N nervous system ; O mouth ; 

 01 olfactory pit ; Ot auditory vesicle (represented as 

 visible) ; Ve velum. 



