VI NEMERTINEA 127 



of hairs, and in this respect is less hke a Ctenophore than the Pihdium 

 larva ; but in having its ciliated band produced into eight processes 

 instead of two, it is more Hke a Ctenophore than the Pilidium. We 

 probably shall not go far astray in concluding that the Pilidium 

 represents a free-swimming ancestor of the Nemertinea, belonging to 

 the same great group as that containing the ancestor of the 

 Ctenophores, but differing from the latter as a shark differs from a 

 salmon, whilst both are fish. 



The metamorphosis into the Nemertine worm must be regarded 

 as the immensely shortened recapitulation of the long development 

 which occurred before this ancestor developed into a Nemertine; a 

 development which must have been much longer than that which 

 was necessary to convert the ancestor denoted by Miiller's larva into 

 the Polyclade worm. Proof will be given as our studies proceed, that 

 such a cataclysmic metamorphosis as that of the Nemertine has been 

 secondarily derived from a type of development that was originally slow 

 and gradual. Nevertheless, as we have indicated above, if this meta- 

 morphosis were thoroughly studied in detail we should know a great 

 deal more about the steps by which that change was accomplished 

 than we do at present. 



LITERATURE REFERRED TO 



Burger. Die Nemertinen. Fauna u. Flora des Golfes von Neapel, vol. 22, 1895. 



MetschnikofF. Studien Tiber die Eutwioklung der Ecliinodermon uud Nemertinen. 

 Mem. Acad. St. Petersb., series 7, vol. 14, 1869. 



Salensky. Bau und Metamorphose des Pilidium. Zeit. fiir wiss. Zool., vol. 43, 

 1886. 



Wilson, C. B. The Habit and Early Development of Cerelraiulus lacteus. Quart. 

 Journ. Micr. Sci., vol. 43, 1900. 



Wilson. E. B. Experiments on Cleavage and Localization in the Nemertine Egg. 

 Arch. Ent-Mech., vol. 16, 1903. 



Yatsu. Experiments on the Development of Egg Fragments in Cerebratulus. Biol. 

 Bull., vol. 6, 1904. 



Zeleny. Experiments on the Localization of Developmental Factors in the Nemertine 

 Egg. Journ. Exp. Zool., vol. 1, 1904. 



