454 CYCLOSTOMATA 



posterior part of the skull, the less specialised head muscles, 

 the lowly organised brain, the primitive nature of the excretory 

 system, and the absence of genital ducts. 



(5) It is probable enough that although the Cyclostomes 

 are not degenerate fishes, they may be in certain respects de- 

 generate, e.g., as regards the eye in Myxinoids or the liver in 

 the lamprey. 



(6) " In the Ammoccete many remarkable features bridge over 

 the gulf between the Craniata and the Cephalochorda. The 

 mouth is bounded by lips ; there is neither sucker nor horny 

 armature, nor yet any rasping " tongue ". The buccal cavity is 

 separated by a velum from the pharynx, and this is limited in 

 front by an encircling ciliated groove (like that of Amphioxus) 

 which is at the level formerly occupied by the transitory first 

 gill-slit. The groove is carried back along the floor of the 

 pharynx into the opening of the thyroid gland. This gland de- 

 velops as a mid-ventral outgrowth of the pharynx, acquires a 

 lumen of considerable size, and along its folded walls become 

 differentiated four rows of mucous cells. In fact, the whole 

 structure bears a striking and unmistakable resemblance to the 

 endostyle of the Tunicata and Cephalochorda, with which it is 

 no doubt homologous " (E. S. Goodrich in Part IX. of Sir Ray 

 Lankester's Treatise on Zoology, 1909, p. $2). 



(7) From the Middle Old Red Sandstone of Caithness, 

 Dr. R. H. Traquair described a minute extinct fish-like 

 creature which he suggested might be a Cyclostome. It is a 

 dainty little creature, somewhat tadpole-like at first sight, 

 usually under an inch in length. The following characters point 

 to affinity with Cyclostomes : — 



(1) "The skull is apparently formed of calcified cartil- 



age, and devoid of discrete ossifications." 



(2) ",Thereis a median opening or ring, surrounded 

 with cirri, and presumably nasal, in front of the 

 head." 



(3) " There are neither jaws nor limbs." 



(4) " The caudal fin is supported by median prolonga- 



tions of the neural and haemal arches, forming rays, 



occasionally forked, very like those of the lamprey." 



But the vertebral column has numerous ring-like centra 



which is very unlike the state of affairs in Cyclostomes, and 



