162 NEMERTEA. 



Rhabdoccela. The development of some of the Rhabdocoela has 

 recently been studied by Hallez. The ova are mostly laid in cap- 

 sules, 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 develope 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 Rhabdoccela 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 lias 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 Prostomuni originates as an 

 epiblastic invagination. 



NEMERTEA. 



Some Nemertea develope 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 



FIG. 91. THREE STAGES IN THE DEVELOPMENT OF LINEUS. (After Barrois. ) 



A is a side view in optical section. 



B and C are two later stages from the ventral (oral) surface. 



ae, archenterou ; sc. segmentation cavity ; //. hypoblast ; me. mesoblast ; ep. epi- 

 blast ; m. mouth ; st. stomach; pr.d. prostomial disc; po.d. metastornial disc; pr. 

 proboscis. 



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

 is therefore without the characteristic appendages of the Pilidium. 



