144 FISHES CHAP. 
these are developed the various parts of the complex adult brain, 
which in the disposition and mutual relations of its parts con- 
forms to a common plan in all the members of the group. There 
are at least ten pairs of cranial nerves having their origin from 
the brain, and, in addition, a varying number of spinal nerves 
arising from the spinal cord, and as a rule formed in each case_ 
by the union of a mainly sensory, ganglionated, dorsal root with 
a mainly motor, non-ganglionated, ventral root. 
The median and usually vestigial, parietal, or pineal eye may 
sometimes be retained as a functional organ, but there exist in all 
Craniates, in addition, paired eyes, the sensory portion of which, 
the retina, is derived as an outgrowth from the first of the 
primary embryonic brain-vesicles. To these organs of special 
sense are added a pair of auditory organs, and a pair of olfactory 
organs, besides, in the lower aquatic Craniates, the peculiar 
sensory organs of the “lateral line.” 
The gonads are reduced to a single pair in the adult, although 
it is possible that they may have a multiple origin in the embryo. 
Gonoducts for the discharge of the sex-cells are almost invariably 
present, and may owe their origin either to a change of function 
on the part of certain kidney-ducts, or to independent evolution 
from the lining membrane of the coelom. The ova are generally 
provided with a large amount of nutritive reserve in the shape 
of food-yolk, and hence the process of segmentation is frequently 
partial or “ meroblastic,” but in some groups, in which the ova 
have less food-yolk, it is complete or “holoblastic.” The typical 
invaginate gastrula stage, which is so striking a feature in the 
embryonic history of the lower Chordata, occurs also in a few of 
the lower Craniates, but in most of them it is apt to become 
masked or modified in various ways by the presence of a super- 
abundant amount of food-yolk. 
Functional hermaphroditism is of very rare occurreuce in 
Craniates, and, as in the Cephalochordata, reproduction by 
budding and the formation of colonies are unknown. 
Thus distinguished from other Chordata, the Craniata are 
divided into six “classes,” which may be variously grouped, as 
the following table shows :— 
