SKULL-LESS ANIMALS. 101 



into two differing main sections, that of the Single-nostrils 

 (Monorhina), and that of the Double-nostrils (Amphirhina). 

 Of the former there are but very few extant forms, which 

 are called Round-mouths (Cyclostoma). These are, however, 

 of great interest, because in their whole structure they are 

 intermediate between the Skull-less Animals and the Double- 

 nostrils (Ampkirhina). Their organization is much higher 

 than that of the Skull-less Animals, much lower than that 

 of the Double-nostrils ; they thus form a very welcome 

 phylogenetic link between those two divisions. We may 

 therefore represent them as a special, tenth stage in the 

 human ancestral series. 



The few existing species of the class of Round-mouths are 

 distributed into two different orders, which are distinguished 

 as the Hags and the Lampreys. The Hags (Myxinoides) 

 have long, cylindrical, worm-like bodies. Linnaeus classed 

 them among Worms, but later zoologists have placed them, 

 sometimes among the Fishes, sometimes Amphibians, and 

 again with Molluscs. The Hags live in the sea and are 

 usually parasitic on Fishes, into the skin of which they 

 penetrate by means of their round sucking mouths and 

 their toothed tongues. They are occasionally found in the 

 body-cavity of Fishes — for example, of the Cod and Stur- 

 geon — having penetrated to the interior in their passage 

 through the skin. The second order, that of the Lampreys 

 (Petromyzontes), includes those well-known " Nine eyes," 

 common at the seaside; the little river Lamprey (Petro- 

 myzon fluviatilis) and the large sea Lamprey (Petromyzon 

 marinus, Fig. 190). 



The animals included in the two groups of the Myxi- 

 noides and the Petromyzontes, are called Round-mouths 



