CHAP. V PLATYHELMINTHES 103 



one could not have a better example of the disturbing influence of a 

 superabundance of food, which we have already seen reason to 

 beUeve is one main cause of variation. 



Now it is obvious that these peculiarities are secondarily- 

 derived, that they are not primary characteristics ; yet they range 

 throughout the entire groups of the Trematoda and Cestoda, and in 

 the group of the Turbellaria they are found in Triclada amongst 

 Dendrocoela, and in EhabdocoeUda and AUoiocoelida amongst 

 Ehabdocoela. In the curiously modified Acoelida, where the gut 

 has no lumen and its cells are indistinguishable from those of the 

 parenchyma, this peculiarity does not occur, nor does it occur in the 

 group Polyclada of the Dendrocoela. 



Now the Acoelida support hfe, as Keeble (1907) has shown, by 

 means of their association with a plant ; this accounts for the 

 absence of a hollow alimentary canal, and it cannot therefore be 

 regarded as a primitive feature. The Polyclada, on the other hand, 

 retain the power of swimming by cilia, and they exhibit other 

 features of a very primitive character ; it is to their ontogeny, there- 

 fore, that we naturally turn for light on the origin of the group. 



The pioneer in this work has been Lang (1889), who has worked at 

 the development of several European genera. We nevertheless take for 

 type the American form Planocera, which has been worked out with 

 great care by Surface (1907), because it is the most recent worlc and 

 the most modern methods have been employed for it, whereas Lang's 

 work dates back twenty to thirty years. Nevertheless the outcome of 

 Surface's work is to support Lang's main conclusions, and to suggest 

 strongly that the eggs of all the genera of Polyclada segment very 

 much in the same way, so that what is here recorded of Planocera 

 will be found to be nearly correct for the European species also. 



PLANOCERA 



Planocera lives in the mantle-cavity of the Gastropod Sycotypus. 

 It does not feed — so far as is known — on the tissues of its host, but 

 uses its host's mantle-cavity as a convenient retreat. It is therefore 

 a commensal, not a parasite. The adults were obtained by opening 

 the branchial cavities of specimens of the Gastropod. They were 

 kept in vessels containing clean sea- water through which a current of 

 air was allowed to bubble. They laid their eggs in these vessels, and 

 the eggs lived till the free-swimming larva escaped. 



The eggs of Planocera are minute, being only -1 mm. in diameter, 

 and they are laid in capsules, usually only one in each capsule, and 

 the capsules are embedded in a gelatinous sUme. Occasionally two 

 eggs are laid in a capsule, and then both become normal embryos. No 

 attempt was made by Surface to extract the eggs from the capsules, 

 but they were examined living and were then preserved in fluids of 

 penetrating power which reached and preserved them whilst still in 

 the capsules. 



