METAZOA VERMES. 315 



chaeta, and Hirudinia), the Gephyrea, Nematodes, and 

 Acanthocephala, may be considered as one division of 

 worms in which the Chaetopods are the most primitive, 

 though at the same time they are examples of specializa- 

 tion by addition, while the Nematodes and Acantho- 

 cephala through the habit of parasitism are the most 

 modified, and offer good illustrations of specialization by 

 reduction. 



The next division, including the Nemertea, Turbellaria, 

 Trematodes, and Cestodes, consists of secondary forms 

 which illustrate in a marked degree acceleration in devel- 

 opment and extreme specialization by the suppression of 

 parts. 



NEMERTEA. 



The Pilidium larva (PL 781, Linens /acteits) peculiar 

 to some Nemertines, has an incomplete digestive system 

 and when older its structure is complicated by having 

 the young worm enclosed within (PI. 782, Pilidium 

 gyrans), the development of which is abbreviated. 1 



Most Nemerteans enclose their eggs in cocoons and 

 the embryo is sometimes protected by a membrane, the 

 amnion. There are various modes of development, but 

 the majority are without a metamorphosis, the develop- 

 ment indicating a concentration of stages. Some are 

 even viviparous, and this process is usually the result of 

 acceleration in development. 



Most Nemerteans are unsegmented though there are 

 some species that show traces of segmentation (Balfour), 

 and the segmented condition is indicated in various sys- 

 tems of organs. 2 



While the young are usually free-swimming, the adults 

 are generally found under stones on the shore. These 



1 Balfour, Comp. Embryol., I, 1880, p. 169. 



2 Gegenbaur, E!em. Comp. Anat., 1878, p. 130. 



