CHAPTER VII. 

 PLATYELMINTHES 1 . 



TURBELLARIA. 



ALTHOUGH there is perhaps no group in the animal kingdom the 

 ontogeny of which would better repay a thorough investigation than 

 the Turbellarians, yet the difficulties to be overcome have hitherto 

 proved too great. 



The fresh-water Rhabdoccela and Dendrocoela do not undergo any 

 metamorphosis, and leave the ovum in a condition in which they 

 cannot easily be distinguished in their general appearance from 

 Infusoria. Many marine Dendrocoela also develop directly, while, 

 as was first shewn by Joh. Miiller, other marine Dendrocoela undergo 

 a more or less complicated metamorphosis. 



Marine Dendrocoela. Of the marine Dendroccela which do not 

 undergo a metamorphosis the form most fully worked out is Lep- 

 toplana tremellaris (vide Keferstein, No. 187, and Hallez, No. 185). 



The ova are surrounded by large albuminous capsules secreted 

 by a special gland. They are laid a great number at a time, and 

 adhere together so as to form masses not unlike the spawn of nudi- 

 branchiate Molluscs. 



Within the egg-capsule the ovum floats freely and undergoes a 

 segmentation similar in many respects to the characteristic molluscan 

 type. The ovum divides into two, and then into four parts, from each 

 of which a small segment is then separated off. The four small 



1 I. Turbellaria. 



1. Deudrocnela. 



2. Rhabdoccela. 



n. Nemertea. 



1. Anopla. 



2. Enopla. 



in. Trematoda. 



1. Distoineae. 



2. Polystomea?. 



iv. Cestoda. 



