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 sensor} 7 , 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 Crauiates, 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 occurrence 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 : 



