i22 VERTEBRATES WITHOUT JAWS iv. 18- 



mainly in size, time of sexual maturity, and habits. A further com- 

 plication is that the germ-cells of the two races allow cross-fertiliza- 

 tion, although this probably never occurs in nature! We may take 

 Dobhzansky's definition of species as 'groups of populations which 

 are reproductively isolated to the extent that the exchange of genes 

 between them is absent or so slow that the genetic differences are not 



Fig. 8i. Myxine, partly dissected. 



i, cloaca; 2, testis; 3 and 4, ovary with eggs; 5, liver; 6, branchial opening; 7, mouth; 

 8, nostril; 9 and 11, slime glands; 10, intestine. (After Retzius, from Kukenthal.) 



diminished or swamped', and in this sense we may retain the specific 

 names L. fluviatilis and L. planeri for the two populations. 



19. Hag-fishes, order Myxinoidea 



The hag-fishes, Myxine and Bdellostoma (Fig. 8 1 ), are animals highly 

 modified for sucking. They live buried in mud or sand and probably eat 

 polychaetes and other invertebrates, as well as scavenging dead fishes. 

 The eyes are functionless rudiments, though the animals are sensitive 

 to changes of illumination, through skin receptors. There are sensory 

 tentacles around the mouth, and in both hag-fishes the teeth and 

 sucking apparatus are well developed. They burrow into the bodies 

 of dead or dying fishes. As many as 123 Myxine have been taken from 

 a single fish. Since the introduction of trawling they have become less 

 common in the North Sea, where they used to be a serious source of 

 loss to fishermen by their attacks on fishes caught in drift nets or on 

 lines. They seem to find fish when they are dying or just dead, and 

 entering by the mouth of their prey eat out the whole contents of the 

 body, leaving a sack of skin and bones. When they are themselves 



