328 MANUAL OF ZOOLOGY sect. 



The central nervous system is a rod-like organ, the neuron or dorsal 

 nerve-cord (Fig. 204), contained within and completely filling a median 

 longitudinal neural canal which lies immediately above the notochord. 

 It is traversed by an axial canal, which becomes dilated at the anterior 

 extremity. From this nerve-cord regularly arranged nerves are given 

 off. 



At the level of the anterior end of the nerve-cord is a narrow ciliated 

 depression, the olfactory pit (Fig. 204, olf. p) opening externally on 

 the left side of the snout and connected at its lower end with a median 

 hollow process of the nerve-cord. This structure is supposed to be an 

 organ of smell. 



The organ of sight is an unpaired pigment spot (<?) in the front wall 

 of the brain ; it is therefore a median eye. A peculiar structure, the 

 groove of Hatschek, on the roof of the oral hood, is supposed to have a 

 sensory function, and may be an organ of taste. Lastly, the sensory 

 cells on the buccal cirri give those organs an important tactile function. 



The sexes are separate, but there is no distinction, apart from the 

 organs of reproduction, between male and female. The gonads (Fig. 

 204, goii) are about twenty-six pairs of pouches arranged metamerically 

 along the body-wall and projecting into the atrium so as largely to fill 

 up its cavity. 



When ripe the inner walls of the gonadic pouches burst, and the ova 

 or sperms make their way into the atrium and thence by the atriopore 

 to the external water, where impregnation takes place. 



B. THE CRANIA TA 



The fishes, amphibians, reptiles, birds, and mammals are 

 grouped together under the general designation Craniata, 

 derived from one of the features which these animals have 

 in common, viz., the presence of a skull or cranium. In 

 order to understand the general characteristics of the Craniata, 

 it will be advisable to examine and compare representatives 

 of some of the principal classes. For this purpose a dogfish, 

 a lizard, 1 and a rabbit will be a good and convenient selection. 



1 The anatomy of the Anolis or " American chameleon " of the Southern 

 States is essentially like that of the European lizard. 



