Vv SYSTEMATIC POSITION yal) 
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 
(Myzine); or, as in the Lampreys (/etromyzon), 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 
hypophysis1 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 (eg. 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 
Tin pa 129: 2 Of spool 
