PLATYELMINTHES. 169 



type and that of Desor, are to be regarded as the primitive. But 

 even in Pilidium there are evidences of a great abbreviation in 

 development. Pilidium itself is probably a more or less modified 

 ancestral form, while the peculiar development of the Nemertine within 

 it is to be explained as a very much shortened record of a long series 

 of changes by which the Pilidium became gradually converted into 

 a Nemertine. The formation of the body wall of the Nemertine by 

 four epiblastic invaginations is a remarkable embryological phenome- 

 non, for which it is not easy to assign a satisfactory meaning; and it is 

 probable that it is merely a secondary process of growth similar to 

 the formation of imaginal discs in the larvae of Diptera (vide Chapter 

 on Tracheata), which has had its origin in the abbreviation of the 

 development just alluded to. The development on the type of Desor 

 is clearly a simplification of the Pilidium type, and its peculiarities 

 are to be explained by the fact that the first larval form has no free 

 existence. The types without metamorphosis have no doubt a de- 

 velopment of a still more simplified character ; they are remarkable 

 however in presenting us, if the existing descriptions are to be trusted, 

 with examples of delamination and invagination coexisting in closely 

 allied forms. 



TREMATODA. 



The eggs of the Trematoda consist of a germ or true ovum 

 enclosed in a mass of yolk cells, which undergo disintegration and 

 subsequent absorption at varying periods of the development. From 

 the observations of E. van Beneden (No. 218), Zeller (No. 217), etc. 

 it is known that the segmentation is usually complete, but generally 

 somewhat irregular. 



Unfortunately we are still completely in the dark as to the mode of 

 formation of the germinal layers. The embryos of the entoparasitic 

 forms or Distomeae become free in a very imperfect condition, and 

 the ova are small; while in the Polystomeae the development is as 

 a rule nearly completed before hatching, and the ova are large. It 

 will be convenient to treat separately the development of the two 

 groups. 



DistomeaB. The embryos of the Distomeae are hatched either in 

 some moist place or more usually in water. In the majority of 

 genera the larvae pass through a complicated metamorphosis, accom- 

 panied by alternations of generations. But for some genera, e.g. 

 Holostomum, etc., the life history has not yet been made out. The 

 whole life history of comparatively few forms has been followed, 

 but sufficient fragments are known to justify us in making certain 

 general statements, which no doubt hold true for a large propor- 

 tion of the Distomeae. 



The larvae are usually ciliated (fig. 95 A), but sometimes naked. 



The ciliated forms are generally completely covered with cilia, but in 

 Distomum lanceolatum the cilia are confined to an area at the front end of 



