800 M. Krohn on the Anatomy and 



all in their state of maturity. According to this_, it is scarcely 

 doubtful that the seminal fluid introduced into the ovarian cavi- 

 ties belongs to them, and that, consequently, the Sagitta fecun- 

 dates itself. But by what means is the seminal fluid transferred 

 from the male into the female apertures, and how does it pass 

 over so great a space ? It is difiicult to say ; I can only ofibr 

 presumptions on this point. If we suppose that the water serves 

 as the vehicle, we are scarcely more advanced, as it is necessary 

 to explain how the seminal fluid is conducted into the ovary. 

 Shall we admit that it is propelled by currents produced by the 

 movements of the vibratory cilia, existing either toward the en- 

 trance into the ovaries or at the mouth of their excretory ducts ? 

 But I have never been able to perceive, in any part, these vibratory 

 cilia in the female genital apparatus. It is therefore probable 

 that the transference of the seminal fluid is eff'ected by the alter- 

 nate approach of the male and female apertures ; and this may 

 take place by means of the tail being curved under the body. 



Nervous system : — Cephalic ganglion. — The principal ganglion 

 of the head, or the cephalic ganglion properly so called, is si- 

 tuated in the middle of the upper surface of the head and at a short 

 distance from its anterior margin, immediately beneath the skin 

 and above the pharynx. It is flattened, nearly hexagonal, and, 

 in adult individuals, it is about a quarter of a millimetre in ex- 

 tent. It sends out two pairs of nerves, one anterior, one poste- 

 rior, and communicates with the trunk or ventral ganglion by 

 two strong and elongated oesophagian commissures. 



Each of the anterior cephalic nerves is detached from the an- 

 terior margin of the ganglion, remains at first almost parallel to 

 its congener, is then directed toward the prominence furnished 

 with bristles already mentioned, penetrates into the fascicles of 

 some muscles, and finally loses itself in the muscle of the hooks 

 of the corresponding side, after having become swelled into a 

 kind of ganglion, in the vicinity of this muscle. From this 

 swelling several filaments radiate, which divide themselves in the 

 muscle. 



The two posterior cephalic nerves, which spring from the pos- 

 terior margin of the ganglion, exhibit remarkable peculiarities. 

 They are larger and more elongated than the anterior ones, re- 

 main, throughout their course, just beneath the skin of the upper 

 surface of the head, and extend to the limits of the trunk. They 

 diverge considerably from their very commencement, and curve 

 finally into a circle toward the median line of the head, where 

 they anastomose, forming a kind of nervous arcade. At a little 

 distance from their origin, each of these small trunks furnishes an 

 optic nerve, which will be considered hereafter. 



Ventral ganglion. — This ganglion is situated in the middle of 



