CHAPTER VI 



NEMEETINEA 



Classification adopted 



Protonemertini 

 Mesonemertini 

 Metanemertini 

 Heteronemertini • 



The group of worms known as the Nemertinea constitute a very 

 interesting division of the animal kingdom. They used to be re- 

 garded as a subdivision of Platyhelminthes, with the more primitive 

 members of which they agree in possessing a soft cihated skin. They 

 also agree with Platyhelminthes in the character of their excretory 

 system and in the general structure of their nervous system, but they 

 differ profoundly from all Platyhelminthes in possessing an anus. 

 The acquisition of a second opening to the alimentary canal and the 

 consequent separation of the functions of ingestion and egestion, or 

 defaecation, is a great structural advance, and is a reason for regarding 

 Nemertinea as standing on a much higher level than Platyhelminthes. 

 The Nemer tinea are divided by Burger (1895) into four groups, 

 Protonemertini, Mesonemertini, Metanemertini, and Heteronemertini. 

 The development of the members of the first two divisions is quite 

 unknown. Metanemertini pass through a shortened development 

 within the egg-shell, the details of which have not been worked out 

 owing to the extremely minute size of the eggs. The majority, how-: 

 ever, of Heteronemertini develop into a free-swimming larva which, 

 after leading a free life in the sea for some time, undergoes a re- 

 markable metamorphosis into the adult worm. There are two types 

 of this larva known, one termed the Pilidium and the other the larva 

 of Desor. But the larva of Desor is quite evidently a simplified 

 form of Pilidium, modified for a creeping rather than for a swimming 

 life, and the Pilidium must be looked upon as the typical larva of the 

 group. 



CEKEBEATULUS LACTEUS 



We select therefore for special description the American species ' 

 Cereiratulus lacteus, the eggs of which develop into a typical Pilidium. 



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