CYCLOSTOMATA. 57 



spinal nerves. The dorsal and caudal fins are supported by 

 paired cartilaginous radialia which are connected proximally 

 with the skeletogenous layer. 



THE SKULL. 



In Myxinoids the cranium is a mere cartilaginous floor 

 without side walls or roof, and the trabeculae 1 end without 

 growing forwards into cornua. In Lampreys the trabeculae 

 grow forwards and send up plates of cartilage which meet 

 above (fig. 4, 7) and form side walls and a roof for part of the 

 brain case. In Lampreys a labial suctorial apparatus is well 

 developed, including a large ring-like piece of cartilage (fig. 4, 2) 

 which supports the oral funnel and bears a large armament of 

 horny teeth. In Myxinoids on the other hand the labial 

 skeleton is small and consists merely of barbels round the 

 mouth. 



The olfactory organ of Myxinoids has a very curious 

 skeleton. It is covered with a kind of grating of cartilage 

 which is prolonged in front into a tube composed of a series of 

 imperfect cartilaginous rings. In Lampreys the olfactory 

 organ opens merely by a short membranous passage. In 

 correlation with the small development of the labial suctorial 

 apparatus in Myxinoids the lingual apparatus is very greatly 

 developed. The tongue in Myxine has been said to 'dominate 

 the whole body' (Parker). It is supported by a great median 

 cartilaginous bar which when followed forwards first becomes 

 bifid and still further forwards becomes four-cleft. 



The horny teeth in Myxinoids are chiefly borne on the 

 very large supralingual apparatus. They form a double series 

 arranged in the form of an arch. In Myxine there are seven 

 large teeth and nine small ones on each side. In Bdellostoma 

 the teeth of the two rows are more equal in size. In Bdello- 

 stoma and Myxine it has been shown that imperfect calcified 

 teeth occur below the horny teeth. 



1 See p. 17. 



