v SYSTEMATIC POSITION 147 



Haeckel that " they are further removed from Fishes than Fishes 

 from Man." 



Briefly stated, the Cyclostomata or Agnathostomata are 

 distinguished from " Fishes " and all the remaining Craniata 

 (Gnathostomata) by the following characters : 



The mouth is either nearly terminal, as in the Hag-Fishes 

 (Myxine) ; or, as in the Lampreys (Petromyzon), it opens out 

 of a spacious, pre-oral, suctorial, buccal funnel, which, in its rela- 

 tions to the hypophysis or pituitary body, recalls the pre-oral 

 buccal cavity of the Cephalochordata. As in Amphioxus, the 

 hypophysis l is displaced dorsally by the forward growth of the 

 pre-oral portion of the head in the embryo, and consequently it 

 only attains its normal relations to the infundibular downgrowth 2 

 from the ventral surface of the fore-brain by perforating the 

 floor of the skull from above instead of from below as in all 

 other Craniates. In one section of the group (e.g. Myxine) the 

 hypophysis opens into the oral cavity, and serves as a tubular 

 passage for the inspiratory water-current to the gill-sacs, a feature 

 in which these Cyclostomes are unique. The apparently median 

 olfactory organ is carried inwards with the hypophysial involution, 

 and communicates with the latter throughout life. A primitive 

 upper jaw (palato-quadrate cartilages or sub-ocular arches) is 

 present, and in at least some Cyclostomes (e.g. the Lampreys), 

 and possibly in all, there are structures which very probably 

 represent a primitive lower jaw (Meckel's cartilages); but such 

 structures are always non-biting, and merely form skeletal supports 

 for other portions of the skull. In place of biting jaws the 

 mouth is provided with a complex rasping lingual apparatus 

 supported by special cartilages, the so-called tongue, which bears 

 horny teeth and is capable of protrusion and retraction. Paired 

 limbs are entirely wanting. 



In the Gnathostomata, on the contrary, there is no buccal 

 funnel, and the mouth, whether terminal or ventral in position, 

 opens directly outwards. The hypophysis is usually carried 

 inwards with the stomatodaeal invagination which in the embryo 

 gives rise to the mouth, and is therefore from the first in relation 

 with the ventral surface of the brain. Biting jaws (palato- 

 quadrate and Meckelian cartilages), formed by the modification 

 of an anterior and primitively gill-bearing visceral arch, are 

 1 Cf. p. 129. 2 Cf. p. 391. 



