116 



NEMERTINEA 



[CH. 





6 



FIG. 49. Cerebratulus fuscus. 

 Young transparent form x 7. 

 After Burger. 



1. Cephalic pits. 2. Opening 

 leading into the retracted pro- 

 boscis. 3. Dorsal commissure 

 of nervous system. 4. Ventral 

 commissure. 5. Brain. 6. Post- 

 erior lobe of brain which comes 

 into connection with the cephalic 

 pit. 7. Mouth. 8. Proboscis 

 sheath. 9. Lateral vessel. 



10. Proboscis. 11. Pouches of 

 alimentary canal. 12. Stomach. 



of these are closely applied to and 

 even indent the wall of the lateral 

 blood-vessel, while in each termina- 

 tion there is a tuft of cilia. 



The Nemertinea have testes and 

 ovaries borne by different indi- 

 viduals, that is to say the sexes are 

 separated. The generative organs 

 are exceedingly simple, consisting 

 in both sexes of packets of cells 

 situated along the sides of the body 

 alternating with the pouches of the 

 alimentary canal. No permanent 

 genital ducts exist, but when the 

 ova and spermatozoa are ripe they 

 appear to make temporary ducts for 

 themselves. The egg develops in 

 many cases into a remarkable larva 

 called a Pilidium. This is shaped 

 something like a policeman's helmet 

 with ear lappets. The edge of the 

 helmet, including the lappets, is 

 fringed with powerful cilia, and there 

 is besides a tuft of long cilia at the 

 apex. Underneath the helmet is the 

 opening of the mouth, which leads 

 into a sac-like gut devoid of an anus. 



The adult is developed from the 

 Pilidium in an extraordinary way. 

 Four invaginations of ectoderm 

 appear on the under side of the 

 larva, at the sides of the alimentary 

 canal. These grow both upwards 

 and inwards until they completely 

 surround the canal. The inner walls 

 of the pockets form the ectoderm 

 of the adult. When the process is 

 complete the alimentary canal sur- 

 rounded by the new ectoderm drops 

 out of the Pilidium and forms the 

 Nernertine. 



