STRUCTURE OF THE ADULT AX1MAL. 43 



central canal of this tube is very small along the whole length 

 of the cord, except at the extreme anterior end, where it expands 

 to form a thin-walled chamber, or ventricle. This dilatation of 

 the central canal constitutes the only indication, if indeed it can 

 be regarded as such, of anything corresponding to the brain of 

 higher Vertebrates. The nerves arise either by single roots 

 from the dorsal surface of the cord, or by multiple roots from 

 its ventral surface : the two sets of nerves, which are quite 

 independent of each other, appear to correspond with the dorsal 

 and ventral roots of the spinal nerves of other Vertebrates, 

 although the dorsal roots have no ganglia and are both sensory 

 and motor in function. Excepting the anterior three or four 

 of each side, the nerves arise, not in pairs, but alternately from 

 the right and left sides of the cord. 



The sense organs are in a very simple condition, and can 

 only doubtfully be compared with those of higher Vertebrates. 

 From the anterior end of the ventricle of the central nervous 

 system, a hollow outgrowth arises, which is in close relation with 

 a ciliated pit on the dorsal surface and left side of the anterior 

 end of the animal. This pit is commonly regarded as an olfac- 

 tory organ. 



The ' eye ' is a rounded pigment-spot in the anterior wall of 

 the ventricle ; i.e. at the anterior end of the central nervous 

 system (Fig. 11, M). There is no trace of an ear. 



The sexes are distinct, but the male and female are similar, 

 except as regards the microscopic structure of the reproductive 

 organs. There are no genital ducts. 



In the female, the ovaries (Fig. 13, ov) are a series of 

 saccular organs, arranged in a row along the inner surface of 

 the body wall, on each side of the pharynx, in the segments from 

 the tenth to the thirty-sixth. They lie in cavities, which are 

 specialised portions of the ccelom, and the true relations of which 

 will be described when the development of the reproductive 

 organs is considered. 



The ova, when ripe, are discharged into the atrial cavity by 

 dehiscence of the proper wall of the ovary and of the atrial 

 membrane. The discharged ova, together with the ovaries, 

 form a bulky mass, which causes great distension of the atrial 

 cavity, and distortion, through pressure, of the pharynx and 

 other organs. 



