196 NEMERTEA. 



gradually loses its hemispherical form, and assumes an elongated and 

 flattened shape. It acquires a coating of cilia by means of which it rotates. 

 On the fifth day it is hatched. 



The alimentary tract long remains solid, even after it has acquired its 

 branched form. The pharynx becomes withdrawn as soon as the larva is 

 hatched. It loses its provisional muscles, and subsequently acquires a 

 permanent musculature. The young after hatching attach themselves to the 

 body of their parent, on which they feed (?). 



Rhabdocoela. The development of some of the Rhabdocoela 

 has recently been studied by Hallez. The ova are mostly laid 

 in capsules, one in each capsule. Sometimes the development 

 commences before the capsules are laid, at other times not till 

 afterwards. In certain forms (Mesostomum) there are summer 

 eggs with thin capsules which develop within the parent, while 

 hard capsules, forming what are known as winter eggs, are laid 

 in the autumn, and the embryo hatched in the spring. 



The ova of the Rhabdocoela like those of the fresh-water Dendroccela 

 are enveloped in yolk elements derived from the vitellarium. 



The segmentation probably takes place in the same way as in Lepto- 

 plana. A stage with four equal cells has been observed by Hallez, and 

 there is subsequently an epibolic gastrula. The embryo becomes ciliated 

 while still within the capsule and, according to Hallez, the pharynx arises 

 as a bud of the hypoblast. The proboscis in Prostomum originates as an 

 epiblastic invagination. 



NEMERTEA. 



Some Nemertea develop without and some with a meta- 

 morphosis. 



The most remarkable type of Nemertine development with a 

 metamorphosis is that in which the ovum develops into a 

 peculiar larval form known as Pilidium, within which the perfect 

 worm is subsequently evolved. Closely allied to this type is one 

 in which the sexual worm is developed within a larval form as in 

 Pilidium, but in which the larva has no free swimming stage, and 

 is therefore without the characteristic appendages of the Pilidium. 

 This is known as the type of Desor and is confined (?) to the 

 genus Lineus. The Pilidium and the Desor type may be first 

 considered (vide Barrois, No. 192). 



The type of Desor. The segmentation is regular and leads 

 to the formation of a blastosphere with a large segmentation 



