CYCLOSTOMATA. 149 



SUB-CLASS I. CARTILAGINEI. 



Skeleton cartilaginous, without bony fibres ; 

 skull formed of a single piece. 

 With fixed gills : 



Mouth forming a sucker .... Cyclostomata. 

 Mouth armed with jaws . . . Selachia. 



With free gills Sturiona. 



Order L CYCL OSTOMA TA. 



Skeleton rudimentary ; spine an unjointed 

 column of cartilage ; no ribs ; no pectoral or ven- 

 tral fins ; mouth a sucking -disk, surrounded by a 

 ring of cartilage. 



219 



Amphioxus (Yarrell). Lancelet. Compressed, 

 doubly pointed ; mouth a long slit, with a row 

 of filaments on each side ; a dorsal fin along the 

 whole length of the back. 



A. lanceolatus. Fig. 217 ; 



n. s. 



